diff --git a/src/plugins/discourse/referenceDetector.js b/src/plugins/discourse/referenceDetector.js
new file mode 100644
index 0000000..214d333
--- /dev/null
+++ b/src/plugins/discourse/referenceDetector.js
@@ -0,0 +1,64 @@
+// @flow
+
+import type {NodeAddressT} from "../../core/graph";
+import type {ReferenceDetector, URL} from "../../core/references";
+import type {ReadRepository} from "./mirrorRepository";
+import {topicAddress, userAddress, postAddress} from "./address";
+import {linksToReferences} from "./references";
+
+/**
+ * Discourse ReferenceDetector detector that relies on database lookups.
+ */
+export class DiscourseReferenceDetector implements ReferenceDetector {
+  data: ReadRepository;
+
+  constructor(data: ReadRepository) {
+    this.data = data;
+  }
+
+  addressFromUrl(url: URL): ?NodeAddressT {
+    const [reference] = linksToReferences([url]);
+    if (!reference) {
+      return null;
+    }
+
+    switch (reference.type) {
+      case "TOPIC": {
+        // Just validating the topic exists.
+        if (this.data.topicById(reference.topicId)) {
+          return topicAddress(reference.serverUrl, reference.topicId);
+        }
+        break;
+      }
+
+      case "POST": {
+        // For posts, we need to convert from topicId + index to a post ID.
+        // We're using it to validate the topic and post index exist as well.
+        const postId = this.data.findPostInTopic(
+          reference.topicId,
+          reference.postIndex
+        );
+        if (postId) {
+          return postAddress(reference.serverUrl, postId);
+        }
+        break;
+      }
+
+      case "USER": {
+        // Look up the username to validate it exists, and make sure we use
+        // the result. As this should correct our capitalization. See #1479.
+        const username = this.data.findUsername(reference.username);
+        if (username) {
+          return userAddress(reference.serverUrl, username);
+        }
+        break;
+      }
+
+      default: {
+        throw new Error(
+          `Unexpected reference type: ${(reference.type: empty)}`
+        );
+      }
+    }
+  }
+}
diff --git a/src/plugins/discourse/referenceDetector.test.js b/src/plugins/discourse/referenceDetector.test.js
new file mode 100644
index 0000000..8176de4
--- /dev/null
+++ b/src/plugins/discourse/referenceDetector.test.js
@@ -0,0 +1,112 @@
+// @flow
+
+import Database from "better-sqlite3";
+import type {Topic, Post} from "./fetch";
+import {SqliteMirrorRepository} from "./mirrorRepository";
+import {DiscourseReferenceDetector} from "./referenceDetector";
+import {type NodeAddressT, NodeAddress} from "../../core/graph";
+
+const TEST_URL = "https://example.com";
+
+const emptyRepository = (): SqliteMirrorRepository => {
+  const db = new Database(":memory:");
+  return new SqliteMirrorRepository(db, TEST_URL);
+};
+
+const maybeToParts = (a: ?NodeAddressT) => {
+  return a ? NodeAddress.toParts(a) : a;
+};
+
+describe("plugins/discourse/referenceDetector", () => {
+  describe("DiscourseReferenceDetector", () => {
+    it("should detect user reference", () => {
+      // Given
+      const repo = emptyRepository();
+      const detector = new DiscourseReferenceDetector(repo);
+      const username = "PascalFan1988";
+      repo.addUser(username);
+
+      // When
+      const result = detector.addressFromUrl(`${TEST_URL}/u/pascalfan1988`);
+
+      // Then
+      expect(maybeToParts(result)).toMatchInlineSnapshot(`
+        Array [
+          "sourcecred",
+          "discourse",
+          "user",
+          "https://example.com",
+          "PascalFan1988",
+        ]
+      `);
+    });
+
+    it("should detect topic reference", () => {
+      // Given
+      const repo = emptyRepository();
+      const detector = new DiscourseReferenceDetector(repo);
+      const topic: Topic = {
+        id: 123,
+        categoryId: 1,
+        title: "Sample topic",
+        timestampMs: 456789,
+        bumpedMs: 456999,
+        authorUsername: "credbot",
+      };
+      repo.addTopic(topic);
+
+      // When
+      const result = detector.addressFromUrl(`${TEST_URL}/t/random-slug/123`);
+
+      // Then
+      expect(maybeToParts(result)).toMatchInlineSnapshot(`
+        Array [
+          "sourcecred",
+          "discourse",
+          "topic",
+          "https://example.com",
+          "123",
+        ]
+      `);
+    });
+
+    it("should detect post reference", () => {
+      // Given
+      const repo = emptyRepository();
+      const detector = new DiscourseReferenceDetector(repo);
+      const topic: Topic = {
+        id: 123,
+        categoryId: 1,
+        title: "Sample topic",
+        timestampMs: 456789,
+        bumpedMs: 456999,
+        authorUsername: "credbot",
+      };
+      const p1: Post = {
+        id: 100,
+        topicId: 123,
+        indexWithinTopic: 1,
+        replyToPostIndex: null,
+        timestampMs: 456789,
+        authorUsername: "credbot",
+        cooked: "<p>Valid post</p>",
+      };
+      repo.addTopic(topic);
+      repo.addPost(p1);
+
+      // When
+      const result = detector.addressFromUrl(`${TEST_URL}/t/random-slug/123/1`);
+
+      // Then
+      expect(maybeToParts(result)).toMatchInlineSnapshot(`
+        Array [
+          "sourcecred",
+          "discourse",
+          "post",
+          "https://example.com",
+          "100",
+        ]
+      `);
+    });
+  });
+});