diff --git a/src/rules/roomEntry.test.ts b/src/rules/roomEntry.test.ts
new file mode 100644
index 0000000..a113ae7
--- /dev/null
+++ b/src/rules/roomEntry.test.ts
@@ -0,0 +1,115 @@
+import { describe, expect, it } from "vitest";
+
+import { sampleContentPack } from "@/data/sampleContentPack";
+
+import { expandLevelFromExit, initializeDungeonLevel } from "./dungeon";
+import { createRoomStateFromTemplate } from "./rooms";
+import { enterRoom } from "./roomEntry";
+
+function createSequenceRoller(values: number[]) {
+  let index = 0;
+
+  return () => {
+    const next = values[index];
+    index += 1;
+    return next;
+  };
+}
+
+describe("room entry flow", () => {
+  it("marks a newly generated room as entered and resolves an empty encounter", () => {
+    const levelState = initializeDungeonLevel({ content: sampleContentPack });
+    const expanded = expandLevelFromExit({
+      content: sampleContentPack,
+      levelState,
+      fromRoomId: "room.level1.start",
+      exitDirection: "north",
+      roomTableCode: "L1SR",
+      roller: createSequenceRoller([1, 1]),
+    });
+
+    const result = enterRoom({
+      content: sampleContentPack,
+      levelState: expanded.levelState,
+      roomId: expanded.createdRoom.id,
+      at: "2026-03-15T12:00:00.000Z",
+    });
+
+    expect(result.firstEntry).toBe(true);
+    expect(result.encounterResolved).toBe(true);
+    expect(result.room.discovery.entered).toBe(true);
+    expect(result.room.discovery.cleared).toBe(true);
+    expect(result.room.encounter?.resultLabel).toBe("No encounter");
+    expect(result.logEntries.map((entry) => entry.text)).toEqual([
+      "Entered Empty Space.",
+      "Empty Space is quiet.",
+    ]);
+  });
+
+  it("resolves a guard encounter once and records the lookup roll", () => {
+    const levelState = initializeDungeonLevel({ content: sampleContentPack });
+    const guardRoom = createRoomStateFromTemplate(
+      sampleContentPack,
+      "room.level1.guard-room",
+      1,
+      "room.level1.normal.guard-room",
+      { x: 0, y: -1 },
+    );
+
+    levelState.rooms[guardRoom.id] = guardRoom;
+    levelState.discoveredRoomOrder.push(guardRoom.id);
+
+    const result = enterRoom({
+      content: sampleContentPack,
+      levelState,
+      roomId: guardRoom.id,
+      roller: createSequenceRoller([6]),
+      at: "2026-03-15T12:00:00.000Z",
+    });
+
+    expect(result.room.templateId).toBe("room.level1.normal.guard-room");
+    expect(result.lookup?.entry.label).toBe("Guard and Warrior");
+    expect(result.room.encounter?.creatureNames).toEqual(["Guard", "Warrior"]);
+    expect(result.room.discovery.cleared).toBe(false);
+    expect(result.logEntries.map((entry) => entry.type)).toEqual(["room", "roll", "room"]);
+    expect(result.logEntries[1]?.text).toContain("Guard and Warrior");
+  });
+
+  it("does not reroll an encounter when re-entering a room", () => {
+    const levelState = initializeDungeonLevel({ content: sampleContentPack });
+    const guardRoom = createRoomStateFromTemplate(
+      sampleContentPack,
+      "room.level1.guard-room",
+      1,
+      "room.level1.normal.guard-room",
+      { x: 0, y: -1 },
+    );
+
+    levelState.rooms[guardRoom.id] = guardRoom;
+    levelState.discoveredRoomOrder.push(guardRoom.id);
+
+    const firstEntry = enterRoom({
+      content: sampleContentPack,
+      levelState,
+      roomId: guardRoom.id,
+      roller: createSequenceRoller([4]),
+      at: "2026-03-15T12:00:00.000Z",
+    });
+    const secondEntry = enterRoom({
+      content: sampleContentPack,
+      levelState: firstEntry.levelState,
+      roomId: guardRoom.id,
+      roller: createSequenceRoller([6]),
+      at: "2026-03-15T12:05:00.000Z",
+    });
+
+    expect(firstEntry.room.encounter?.resultLabel).toBe("Guard");
+    expect(secondEntry.room.encounter?.resultLabel).toBe("Guard");
+    expect(secondEntry.lookup).toBeUndefined();
+    expect(secondEntry.firstEntry).toBe(false);
+    expect(secondEntry.logEntries.map((entry) => entry.text)).toEqual([
+      "Re-entered Guard Room.",
+      "Encounter in Guard Room: Guard.",
+    ]);
+  });
+});
diff --git a/src/rules/roomEntry.ts b/src/rules/roomEntry.ts
new file mode 100644
index 0000000..e649d98
--- /dev/null
+++ b/src/rules/roomEntry.ts
@@ -0,0 +1,164 @@
+import { findRoomTemplateById } from "@/data/contentHelpers";
+import type { ContentPack } from "@/types/content";
+import type { DungeonLevelState, RoomState } from "@/types/state";
+import type { LogEntry } from "@/types/rules";
+
+import type { DiceRoller } from "./dice";
+import { resolveRoomEncounter } from "./encounters";
+import type { TableLookupResult } from "./tables";
+
+export type EnterRoomOptions = {
+  content: ContentPack;
+  levelState: DungeonLevelState;
+  roomId: string;
+  roller?: DiceRoller;
+  at?: string;
+};
+
+export type EnterRoomResult = {
+  levelState: DungeonLevelState;
+  room: RoomState;
+  logEntries: LogEntry[];
+  lookup?: TableLookupResult;
+  firstEntry: boolean;
+  encounterResolved: boolean;
+};
+
+function cloneRoom(room: RoomState): RoomState {
+  return {
+    ...room,
+    position: { ...room.position },
+    dimensions: { ...room.dimensions },
+    exits: room.exits.map((exit) => ({ ...exit })),
+    discovery: { ...room.discovery },
+    encounter: room.encounter
+      ? {
+          ...room.encounter,
+          creatureIds: [...room.encounter.creatureIds],
+          creatureNames: room.encounter.creatureNames
+            ? [...room.encounter.creatureNames]
+            : undefined,
+        }
+      : undefined,
+    objects: room.objects.map((object) => ({
+      ...object,
+      effects: object.effects ? [...object.effects] : undefined,
+    })),
+    notes: [...room.notes],
+    flags: [...room.flags],
+  };
+}
+
+function cloneLevel(levelState: DungeonLevelState): DungeonLevelState {
+  return {
+    ...levelState,
+    rooms: Object.fromEntries(
+      Object.entries(levelState.rooms).map(([roomId, room]) => [roomId, cloneRoom(room)]),
+    ),
+    discoveredRoomOrder: [...levelState.discoveredRoomOrder],
+  };
+}
+
+function getRoomLabel(content: ContentPack, room: RoomState) {
+  if (room.templateId) {
+    return findRoomTemplateById(content, room.templateId).title;
+  }
+
+  return room.notes[0] ?? room.id;
+}
+
+function createLogEntry(
+  id: string,
+  at: string,
+  type: LogEntry["type"],
+  text: string,
+  relatedIds: string[],
+): LogEntry {
+  return {
+    id,
+    at,
+    type,
+    text,
+    relatedIds,
+  };
+}
+
+function formatRollText(lookup: TableLookupResult) {
+  const total = lookup.roll.modifiedTotal ?? lookup.roll.total;
+  const rolledValues = lookup.roll.rolls.join(", ");
+
+  return `Rolled ${lookup.roll.diceKind} [${rolledValues}] on ${lookup.tableId} for ${total}: ${lookup.entry.label}.`;
+}
+
+export function enterRoom(options: EnterRoomOptions): EnterRoomResult {
+  const levelState = cloneLevel(options.levelState);
+  const room = levelState.rooms[options.roomId];
+
+  if (!room) {
+    throw new Error(`Unknown room id: ${options.roomId}`);
+  }
+
+  const at = options.at ?? new Date().toISOString();
+  const roomLabel = getRoomLabel(options.content, room);
+  const firstEntry = !room.discovery.entered;
+  room.discovery.entered = true;
+
+  const resolution = resolveRoomEncounter(options.content, room, options.roller);
+  const nextRoom = resolution.room;
+  const encounterResolved = Boolean(resolution.encounter);
+
+  if (resolution.encounter && resolution.encounter.creatureIds.length === 0) {
+    nextRoom.discovery.cleared = true;
+  }
+
+  levelState.rooms[nextRoom.id] = nextRoom;
+
+  const entryVerb = firstEntry ? "Entered" : "Re-entered";
+  const logEntries: LogEntry[] = [
+    createLogEntry(
+      `${nextRoom.id}.entry.${firstEntry ? "first" : "repeat"}`,
+      at,
+      "room",
+      `${entryVerb} ${roomLabel}.`,
+      [nextRoom.id],
+    ),
+  ];
+
+  if (resolution.lookup) {
+    logEntries.push(
+      createLogEntry(
+        `${nextRoom.id}.entry.roll`,
+        at,
+        "roll",
+        formatRollText(resolution.lookup),
+        [nextRoom.id, resolution.lookup.tableId],
+      ),
+    );
+  }
+
+  if (resolution.encounter) {
+    const encounterText =
+      resolution.encounter.creatureIds.length === 0
+        ? `${roomLabel} is quiet.`
+        : `Encounter in ${roomLabel}: ${resolution.encounter.resultLabel ?? "Unknown threat"}.`;
+
+    logEntries.push(
+      createLogEntry(
+        `${nextRoom.id}.entry.encounter`,
+        at,
+        "room",
+        encounterText,
+        [nextRoom.id, resolution.encounter.id],
+      ),
+    );
+  }
+
+  return {
+    levelState,
+    room: nextRoom,
+    logEntries,
+    lookup: resolution.lookup,
+    firstEntry,
+    encounterResolved,
+  };
+}
diff --git a/src/rules/rooms.ts b/src/rules/rooms.ts
index d581be7..7b0228c 100644
--- a/src/rules/rooms.ts
+++ b/src/rules/rooms.ts
@@ -136,7 +136,9 @@ export function createRoomStateFromTemplate(
     },
     encounter: undefined,
     objects: [],
-    notes: [template.text ?? template.title].filter(Boolean),
+    notes: [template.text ?? template.title, template.encounterText].filter(
+      (note): note is string => Boolean(note),
+    ),
     flags: [
       `table:${template.tableCode}`,
       `entry:${template.tableEntryKey}`,