diff --git a/plonky2/src/gadgets/select.rs b/plonky2/src/gadgets/select.rs
index db9be843..d234a003 100644
--- a/plonky2/src/gadgets/select.rs
+++ b/plonky2/src/gadgets/select.rs
@@ -32,9 +32,8 @@ impl<F: RichField + Extendable<D>, const D: usize> CircuitBuilder<F, D> {
 
     /// See `select_ext`.
     pub fn select(&mut self, b: BoolTarget, x: Target, y: Target) -> Target {
-        let x_ext = self.convert_to_ext(x);
-        let y_ext = self.convert_to_ext(y);
-        self.select_ext(b, x_ext, y_ext).to_target_array()[0]
+        let tmp = self.mul_sub(b.target, y, y);
+        self.mul_sub(b.target, x, tmp)
     }
 }
 
diff --git a/plonky2/src/plonk/conditional_recursive_verifier.rs b/plonky2/src/plonk/conditional_recursive_verifier.rs
new file mode 100644
index 00000000..8d5b2e88
--- /dev/null
+++ b/plonky2/src/plonk/conditional_recursive_verifier.rs
@@ -0,0 +1,353 @@
+use itertools::Itertools;
+use plonky2_field::extension::Extendable;
+
+use crate::fri::proof::{
+    FriInitialTreeProofTarget, FriProofTarget, FriQueryRoundTarget, FriQueryStepTarget,
+};
+use crate::gadgets::polynomial::PolynomialCoeffsExtTarget;
+use crate::hash::hash_types::{HashOutTarget, MerkleCapTarget, RichField};
+use crate::hash::merkle_proofs::MerkleProofTarget;
+use crate::iop::ext_target::ExtensionTarget;
+use crate::iop::target::{BoolTarget, Target};
+use crate::plonk::circuit_builder::CircuitBuilder;
+use crate::plonk::circuit_data::{CommonCircuitData, VerifierCircuitTarget};
+use crate::plonk::config::{AlgebraicHasher, GenericConfig};
+use crate::plonk::proof::{OpeningSetTarget, ProofTarget, ProofWithPublicInputsTarget};
+use crate::with_context;
+
+impl<F: RichField + Extendable<D>, const D: usize> CircuitBuilder<F, D> {
+    /// Verify `proof0` if `condition` else verify `proof1`.
+    /// `proof0` and `proof1` are assumed to use the same `CommonCircuitData`.
+    pub fn conditionally_verify_proof<C: GenericConfig<D, F = F>>(
+        &mut self,
+        condition: BoolTarget,
+        proof_with_pis0: ProofWithPublicInputsTarget<D>,
+        inner_verifier_data0: &VerifierCircuitTarget,
+        proof_with_pis1: ProofWithPublicInputsTarget<D>,
+        inner_verifier_data1: &VerifierCircuitTarget,
+        inner_common_data: &CommonCircuitData<F, C, D>,
+    ) where
+        C::Hasher: AlgebraicHasher<F>,
+    {
+        let ProofWithPublicInputsTarget {
+            proof:
+                ProofTarget {
+                    wires_cap: wires_cap0,
+                    plonk_zs_partial_products_cap: plonk_zs_partial_products_cap0,
+                    quotient_polys_cap: quotient_polys_cap0,
+                    openings: openings0,
+                    opening_proof: opening_proof0,
+                },
+            public_inputs: public_inputs0,
+        } = proof_with_pis0;
+        let ProofWithPublicInputsTarget {
+            proof:
+                ProofTarget {
+                    wires_cap: wires_cap1,
+                    plonk_zs_partial_products_cap: plonk_zs_partial_products_cap1,
+                    quotient_polys_cap: quotient_polys_cap1,
+                    openings: openings1,
+                    opening_proof: opening_proof1,
+                },
+            public_inputs: public_inputs1,
+        } = proof_with_pis1;
+
+        let selected_proof = with_context!(self, "select proof", {
+            let selected_wires_cap = self.select_cap(condition, wires_cap0, wires_cap1);
+            let selected_plonk_zs_partial_products_cap = self.select_cap(
+                condition,
+                plonk_zs_partial_products_cap0,
+                plonk_zs_partial_products_cap1,
+            );
+            let selected_quotient_polys_cap =
+                self.select_cap(condition, quotient_polys_cap0, quotient_polys_cap1);
+            let selected_openings = self.select_opening_set(condition, openings0, openings1);
+            let selected_opening_proof =
+                self.select_opening_proof(condition, opening_proof0, opening_proof1);
+            let selected_public_inputs = self.select_vec(condition, public_inputs0, public_inputs1);
+            ProofWithPublicInputsTarget {
+                proof: ProofTarget {
+                    wires_cap: selected_wires_cap,
+                    plonk_zs_partial_products_cap: selected_plonk_zs_partial_products_cap,
+                    quotient_polys_cap: selected_quotient_polys_cap,
+                    openings: selected_openings,
+                    opening_proof: selected_opening_proof,
+                },
+                public_inputs: selected_public_inputs,
+            }
+        });
+        let selected_verifier_data = VerifierCircuitTarget {
+            constants_sigmas_cap: self.select_cap(
+                condition,
+                inner_verifier_data0.constants_sigmas_cap.clone(),
+                inner_verifier_data1.constants_sigmas_cap.clone(),
+            ),
+        };
+
+        self.verify_proof(selected_proof, &selected_verifier_data, inner_common_data);
+    }
+
+    fn select_vec(&mut self, b: BoolTarget, v0: Vec<Target>, v1: Vec<Target>) -> Vec<Target> {
+        v0.into_iter()
+            .zip_eq(v1)
+            .map(|(t0, t1)| self.select(b, t0, t1))
+            .collect()
+    }
+
+    fn select_hash(
+        &mut self,
+        b: BoolTarget,
+        h0: HashOutTarget,
+        h1: HashOutTarget,
+    ) -> HashOutTarget {
+        HashOutTarget {
+            elements: std::array::from_fn(|i| self.select(b, h0.elements[i], h1.elements[i])),
+        }
+    }
+
+    fn select_cap(
+        &mut self,
+        b: BoolTarget,
+        cap0: MerkleCapTarget,
+        cap1: MerkleCapTarget,
+    ) -> MerkleCapTarget {
+        assert_eq!(cap0.0.len(), cap1.0.len());
+        MerkleCapTarget(
+            cap0.0
+                .into_iter()
+                .zip_eq(cap1.0)
+                .map(|(h0, h1)| self.select_hash(b, h0, h1))
+                .collect(),
+        )
+    }
+
+    fn select_vec_cap(
+        &mut self,
+        b: BoolTarget,
+        v0: Vec<MerkleCapTarget>,
+        v1: Vec<MerkleCapTarget>,
+    ) -> Vec<MerkleCapTarget> {
+        v0.into_iter()
+            .zip_eq(v1)
+            .map(|(c0, c1)| self.select_cap(b, c0, c1))
+            .collect()
+    }
+
+    fn select_opening_set(
+        &mut self,
+        b: BoolTarget,
+        os0: OpeningSetTarget<D>,
+        os1: OpeningSetTarget<D>,
+    ) -> OpeningSetTarget<D> {
+        OpeningSetTarget {
+            constants: self.select_vec_ext(b, os0.constants, os1.constants),
+            plonk_sigmas: self.select_vec_ext(b, os0.plonk_sigmas, os1.plonk_sigmas),
+            wires: self.select_vec_ext(b, os0.wires, os1.wires),
+            plonk_zs: self.select_vec_ext(b, os0.plonk_zs, os1.plonk_zs),
+            plonk_zs_next: self.select_vec_ext(b, os0.plonk_zs_next, os1.plonk_zs_next),
+            partial_products: self.select_vec_ext(b, os0.partial_products, os1.partial_products),
+            quotient_polys: self.select_vec_ext(b, os0.quotient_polys, os1.quotient_polys),
+        }
+    }
+
+    fn select_vec_ext(
+        &mut self,
+        b: BoolTarget,
+        v0: Vec<ExtensionTarget<D>>,
+        v1: Vec<ExtensionTarget<D>>,
+    ) -> Vec<ExtensionTarget<D>> {
+        v0.into_iter()
+            .zip_eq(v1)
+            .map(|(e0, e1)| self.select_ext(b, e0, e1))
+            .collect()
+    }
+
+    fn select_opening_proof(
+        &mut self,
+        b: BoolTarget,
+        proof0: FriProofTarget<D>,
+        proof1: FriProofTarget<D>,
+    ) -> FriProofTarget<D> {
+        FriProofTarget {
+            commit_phase_merkle_caps: self.select_vec_cap(
+                b,
+                proof0.commit_phase_merkle_caps,
+                proof1.commit_phase_merkle_caps,
+            ),
+            query_round_proofs: self.select_vec_query_round(
+                b,
+                proof0.query_round_proofs,
+                proof1.query_round_proofs,
+            ),
+            final_poly: PolynomialCoeffsExtTarget(self.select_vec_ext(
+                b,
+                proof0.final_poly.0,
+                proof1.final_poly.0,
+            )),
+            pow_witness: self.select(b, proof0.pow_witness, proof1.pow_witness),
+        }
+    }
+
+    fn select_query_round(
+        &mut self,
+        b: BoolTarget,
+        qr0: FriQueryRoundTarget<D>,
+        qr1: FriQueryRoundTarget<D>,
+    ) -> FriQueryRoundTarget<D> {
+        FriQueryRoundTarget {
+            initial_trees_proof: self.select_initial_tree_proof(
+                b,
+                qr0.initial_trees_proof,
+                qr1.initial_trees_proof,
+            ),
+            steps: self.select_vec_query_step(b, qr0.steps, qr1.steps),
+        }
+    }
+
+    fn select_vec_query_round(
+        &mut self,
+        b: BoolTarget,
+        v0: Vec<FriQueryRoundTarget<D>>,
+        v1: Vec<FriQueryRoundTarget<D>>,
+    ) -> Vec<FriQueryRoundTarget<D>> {
+        v0.into_iter()
+            .zip_eq(v1)
+            .map(|(qr0, qr1)| self.select_query_round(b, qr0, qr1))
+            .collect()
+    }
+
+    fn select_initial_tree_proof(
+        &mut self,
+        b: BoolTarget,
+        proof0: FriInitialTreeProofTarget,
+        proof1: FriInitialTreeProofTarget,
+    ) -> FriInitialTreeProofTarget {
+        FriInitialTreeProofTarget {
+            evals_proofs: proof0
+                .evals_proofs
+                .into_iter()
+                .zip_eq(proof1.evals_proofs)
+                .map(|((v0, p0), (v1, p1))| {
+                    (
+                        self.select_vec(b, v0, v1),
+                        self.select_merkle_proof(b, p0, p1),
+                    )
+                })
+                .collect(),
+        }
+    }
+
+    fn select_merkle_proof(
+        &mut self,
+        b: BoolTarget,
+        proof0: MerkleProofTarget,
+        proof1: MerkleProofTarget,
+    ) -> MerkleProofTarget {
+        MerkleProofTarget {
+            siblings: proof0
+                .siblings
+                .into_iter()
+                .zip_eq(proof1.siblings)
+                .map(|(h0, h1)| self.select_hash(b, h0, h1))
+                .collect(),
+        }
+    }
+
+    fn select_query_step(
+        &mut self,
+        b: BoolTarget,
+        qs0: FriQueryStepTarget<D>,
+        qs1: FriQueryStepTarget<D>,
+    ) -> FriQueryStepTarget<D> {
+        FriQueryStepTarget {
+            evals: self.select_vec_ext(b, qs0.evals, qs1.evals),
+            merkle_proof: self.select_merkle_proof(b, qs0.merkle_proof, qs1.merkle_proof),
+        }
+    }
+
+    fn select_vec_query_step(
+        &mut self,
+        b: BoolTarget,
+        v0: Vec<FriQueryStepTarget<D>>,
+        v1: Vec<FriQueryStepTarget<D>>,
+    ) -> Vec<FriQueryStepTarget<D>> {
+        v0.into_iter()
+            .zip_eq(v1)
+            .map(|(qs0, qs1)| self.select_query_step(b, qs0, qs1))
+            .collect()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use anyhow::Result;
+
+    use super::*;
+    use crate::field::types::Field;
+    use crate::gates::noop::NoopGate;
+    use crate::iop::witness::{PartialWitness, Witness};
+    use crate::plonk::circuit_data::CircuitConfig;
+    use crate::plonk::config::{GenericConfig, PoseidonGoldilocksConfig};
+
+    #[test]
+    fn test_conditional_recursive_verifier() -> Result<()> {
+        init_logger();
+        const D: usize = 2;
+        type C = PoseidonGoldilocksConfig;
+        type F = <C as GenericConfig<D>>::F;
+        let config = CircuitConfig::standard_recursion_config();
+
+        let mut builder = CircuitBuilder::<F, D>::new(config.clone());
+        let mut pw = PartialWitness::new();
+        let t = builder.add_virtual_target();
+        pw.set_target(t, F::rand());
+        builder.register_public_input(t);
+        let _t2 = builder.square(t);
+        for _ in 0..64 {
+            builder.add_gate(NoopGate, vec![]);
+        }
+        let data = builder.build::<C>();
+        let proof = data.prove(pw)?;
+        data.verify(proof.clone())?;
+
+        let mut builder = CircuitBuilder::<F, D>::new(config);
+        let mut pw = PartialWitness::new();
+        let pt = builder.add_virtual_proof_with_pis(&data.common);
+        pw.set_proof_with_pis_target(&pt, &proof);
+        let dummy_pt = builder.add_virtual_proof_with_pis(&data.common);
+        pw.set_proof_with_pis_target(&dummy_pt, &proof);
+
+        let inner_data = VerifierCircuitTarget {
+            constants_sigmas_cap: builder.add_virtual_cap(data.common.config.fri_config.cap_height),
+        };
+        pw.set_cap_target(
+            &inner_data.constants_sigmas_cap,
+            &data.verifier_only.constants_sigmas_cap,
+        );
+        let dummy_inner_data = VerifierCircuitTarget {
+            constants_sigmas_cap: builder.add_virtual_cap(data.common.config.fri_config.cap_height),
+        };
+        pw.set_cap_target(
+            &dummy_inner_data.constants_sigmas_cap,
+            &data.verifier_only.constants_sigmas_cap,
+        );
+        let b = builder.constant_bool(F::rand().0 % 2 == 0);
+        builder.conditionally_verify_proof(
+            b,
+            pt,
+            &inner_data,
+            dummy_pt,
+            &dummy_inner_data,
+            &data.common,
+        );
+
+        builder.print_gate_counts(100);
+        let data = builder.build::<C>();
+        let proof = data.prove(pw)?;
+        data.verify(proof)
+    }
+
+    fn init_logger() {
+        let _ = env_logger::builder().format_timestamp(None).try_init();
+    }
+}
diff --git a/plonky2/src/plonk/mod.rs b/plonky2/src/plonk/mod.rs
index 73e6c96e..8cd7443f 100644
--- a/plonky2/src/plonk/mod.rs
+++ b/plonky2/src/plonk/mod.rs
@@ -1,5 +1,6 @@
 pub mod circuit_builder;
 pub mod circuit_data;
+pub mod conditional_recursive_verifier;
 pub mod config;
 pub(crate) mod copy_constraint;
 mod get_challenges;