diff --git a/src/main/kotlin/kr/co/vividnext/sodalive/chat/room/CharacterChatParticipant.kt b/src/main/kotlin/kr/co/vividnext/sodalive/chat/room/CharacterChatParticipant.kt
index 60e1d2c..fdfc436 100644
--- a/src/main/kotlin/kr/co/vividnext/sodalive/chat/room/CharacterChatParticipant.kt
+++ b/src/main/kotlin/kr/co/vividnext/sodalive/chat/room/CharacterChatParticipant.kt
@@ -29,7 +29,7 @@ class CharacterChatParticipant(
     @JoinColumn(name = "character_id")
     val character: ChatCharacter? = null,
 
-    val isActive: Boolean = true
+    var isActive: Boolean = true
 ) : BaseEntity() {
     @OneToMany(mappedBy = "participant", cascade = [CascadeType.ALL], fetch = FetchType.LAZY)
     val messages: MutableList<CharacterChatMessage> = mutableListOf()
diff --git a/src/main/kotlin/kr/co/vividnext/sodalive/chat/room/controller/ChatRoomController.kt b/src/main/kotlin/kr/co/vividnext/sodalive/chat/room/controller/ChatRoomController.kt
index d36aa54..1a02229 100644
--- a/src/main/kotlin/kr/co/vividnext/sodalive/chat/room/controller/ChatRoomController.kt
+++ b/src/main/kotlin/kr/co/vividnext/sodalive/chat/room/controller/ChatRoomController.kt
@@ -78,4 +78,23 @@ class ChatRoomController(
         val isActive = chatRoomService.isMyRoomSessionActive(member, chatRoomId)
         ApiResponse.ok(isActive)
     }
+
+    /**
+     * 채팅방 나가기 API
+     * - URL에 chatRoomId 포함
+     * - 내가 참여 중인지 확인 (아니면 "잘못된 접근입니다")
+     * - 내 참여자 isActive=false 처리
+     * - 내가 마지막 USER였다면 외부 API로 세션 종료 호출
+     */
+    @PostMapping("/{chatRoomId}/leave")
+    fun leaveChatRoom(
+        @AuthenticationPrincipal(expression = "#this == 'anonymousUser' ? null : member") member: Member?,
+        @PathVariable chatRoomId: Long
+    ) = run {
+        if (member == null) throw SodaException("로그인 정보를 확인해주세요.")
+        if (member.auth == null) throw SodaException("본인인증을 하셔야 합니다.")
+
+        chatRoomService.leaveChatRoom(member, chatRoomId)
+        ApiResponse.ok(true)
+    }
 }
diff --git a/src/main/kotlin/kr/co/vividnext/sodalive/chat/room/repository/CharacterChatParticipantRepository.kt b/src/main/kotlin/kr/co/vividnext/sodalive/chat/room/repository/CharacterChatParticipantRepository.kt
index 77b2a5b..edd5e60 100644
--- a/src/main/kotlin/kr/co/vividnext/sodalive/chat/room/repository/CharacterChatParticipantRepository.kt
+++ b/src/main/kotlin/kr/co/vividnext/sodalive/chat/room/repository/CharacterChatParticipantRepository.kt
@@ -1,8 +1,8 @@
 package kr.co.vividnext.sodalive.chat.room.repository
 
-import kr.co.vividnext.sodalive.chat.character.ChatCharacter
 import kr.co.vividnext.sodalive.chat.room.CharacterChatParticipant
 import kr.co.vividnext.sodalive.chat.room.CharacterChatRoom
+import kr.co.vividnext.sodalive.chat.room.ParticipantType
 import kr.co.vividnext.sodalive.member.Member
 import org.springframework.data.jpa.repository.JpaRepository
 import org.springframework.stereotype.Repository
@@ -12,10 +12,6 @@ interface CharacterChatParticipantRepository : JpaRepository<CharacterChatPartic
 
     /**
      * 특정 채팅방에 참여 중인 멤버 참여자 찾기
-     *
-     * @param chatRoom 채팅방
-     * @param member 멤버
-     * @return 채팅방 참여자 (없으면 null)
      */
     fun findByChatRoomAndMemberAndIsActiveTrue(
         chatRoom: CharacterChatRoom,
@@ -23,14 +19,10 @@ interface CharacterChatParticipantRepository : JpaRepository<CharacterChatPartic
     ): CharacterChatParticipant?
 
     /**
-     * 특정 채팅방에 참여 중인 캐릭터 참여자 찾기
-     *
-     * @param chatRoom 채팅방
-     * @param character 캐릭터
-     * @return 채팅방 참여자 (없으면 null)
+     * 특정 채팅방의 활성 USER 참여자 수
      */
-    fun findByChatRoomAndCharacterAndIsActiveTrue(
+    fun countByChatRoomAndParticipantTypeAndIsActiveTrue(
         chatRoom: CharacterChatRoom,
-        character: ChatCharacter
-    ): CharacterChatParticipant?
+        participantType: ParticipantType
+    ): Long
 }
diff --git a/src/main/kotlin/kr/co/vividnext/sodalive/chat/room/service/ChatRoomService.kt b/src/main/kotlin/kr/co/vividnext/sodalive/chat/room/service/ChatRoomService.kt
index 7aebc3b..d887859 100644
--- a/src/main/kotlin/kr/co/vividnext/sodalive/chat/room/service/ChatRoomService.kt
+++ b/src/main/kotlin/kr/co/vividnext/sodalive/chat/room/service/ChatRoomService.kt
@@ -15,6 +15,7 @@ import kr.co.vividnext.sodalive.chat.room.repository.CharacterChatParticipantRep
 import kr.co.vividnext.sodalive.chat.room.repository.CharacterChatRoomRepository
 import kr.co.vividnext.sodalive.common.SodaException
 import kr.co.vividnext.sodalive.member.Member
+import org.slf4j.LoggerFactory
 import org.springframework.beans.factory.annotation.Value
 import org.springframework.http.HttpEntity
 import org.springframework.http.HttpHeaders
@@ -45,6 +46,7 @@ class ChatRoomService(
     @Value("\${cloud.aws.cloud-front.host}")
     private val imageHost: String
 ) {
+    private val log = LoggerFactory.getLogger(ChatRoomService::class.java)
 
     /**
      * 채팅방 생성 또는 조회
@@ -250,4 +252,76 @@ class ChatRoomService(
             throw SodaException("오류가 발생했습니다. 다시 시도해 주세요.")
         }
     }
+
+    @Transactional
+    fun leaveChatRoom(member: Member, chatRoomId: Long) {
+        val room = chatRoomRepository.findById(chatRoomId).orElseThrow {
+            SodaException("채팅방을 찾을 수 없습니다.")
+        }
+        val participant = participantRepository.findByChatRoomAndMemberAndIsActiveTrue(room, member)
+            ?: throw SodaException("잘못된 접근입니다")
+
+        // 1) 나가기 처리
+        participant.isActive = false
+        participantRepository.save(participant)
+
+        // 2) 남은 USER 참여자 수 확인
+        val userCount = participantRepository.countByChatRoomAndParticipantTypeAndIsActiveTrue(
+            room,
+            ParticipantType.USER
+        )
+
+        // 3) 내가 마지막 USER였다면 외부 세션 종료
+        if (userCount == 0L) {
+            endExternalSession(room.sessionId)
+        }
+    }
+
+    private fun endExternalSession(sessionId: String) {
+        // 사용자 흐름을 방해하지 않기 위해 실패 시 예외를 던지지 않고 내부 재시도 후 로그만 남깁니다.
+        val maxAttempts = 3
+        var attempt = 0
+        while (attempt < maxAttempts) {
+            attempt++
+            try {
+                val factory = SimpleClientHttpRequestFactory()
+                factory.setConnectTimeout(20000)
+                factory.setReadTimeout(20000)
+
+                val restTemplate = RestTemplate(factory)
+
+                val headers = HttpHeaders()
+                headers.set("x-api-key", apiKey)
+                headers.contentType = MediaType.APPLICATION_JSON
+
+                val httpEntity = HttpEntity(null, headers)
+
+                val response = restTemplate.exchange(
+                    "$apiUrl/api/session/$sessionId/end",
+                    HttpMethod.PUT,
+                    httpEntity,
+                    String::class.java
+                )
+
+                val objectMapper = ObjectMapper()
+                val node = objectMapper.readTree(response.body)
+                val success = node.get("success")?.asBoolean(false) ?: false
+                if (success) {
+                    log.info("[chat] 외부 세션 종료 성공: sessionId={}, attempt={}", sessionId, attempt)
+                    return
+                } else {
+                    log.warn(
+                        "[chat] 외부 세션 종료 응답 실패: sessionId={}, attempt={}, body={}",
+                        sessionId,
+                        attempt,
+                        response.body
+                    )
+                }
+            } catch (e: Exception) {
+                log.warn("[chat] 외부 세션 종료 중 예외: sessionId={}, attempt={}, message={}", sessionId, attempt, e.message)
+            }
+        }
+        // 최종 실패 로그 (예외 미전파)
+        log.error("[chat] 외부 세션 종료 최종 실패: sessionId={}, attempts={}", sessionId, maxAttempts)
+    }
 }