diff --git a/changelog.d/20354.bugfix b/changelog.d/20354.bugfix
new file mode 100644
index 0000000000..6777f2f9ae
--- /dev/null
+++ b/changelog.d/20354.bugfix
@@ -0,0 +1 @@
+Fix Synapse failing to start with Redis enabled when installed using pip (avoid incompatible breaking changes in `txredisapi@1.6.0`).
diff --git a/poetry.lock b/poetry.lock
index a19babbcd9..2649e2450d 100644
--- a/poetry.lock
+++ b/poetry.lock
@@ -1972,7 +1972,7 @@ files = [
     {file = "pycparser-3.0-py3-none-any.whl", hash = "sha256:b727414169a36b7d524c1c3e31839a521725078d7b2ff038656844266160a992"},
     {file = "pycparser-3.0.tar.gz", hash = "sha256:600f49d217304a5902ac3c37e1281c9fe94e4d0489de643a9504c5cdfdfc6b29"},
 ]
-markers = {main = "implementation_name != \"PyPy\"", dev = "platform_python_implementation != \"PyPy\" and implementation_name != \"PyPy\""}
+markers = {main = "implementation_name != \"PyPy\"", dev = "implementation_name != \"PyPy\" and platform_python_implementation != \"PyPy\""}
 
 [[package]]
 name = "pydantic"
@@ -3661,4 +3661,4 @@ url-preview = ["beautifulsoup4"]
 [metadata]
 lock-version = "2.1"
 python-versions = ">=3.10.0,<4.0.0"
-content-hash = "b8ee2c063150549de2946f70cd557c427e16a9101052915ef23eb6dd9d8f77eb"
+content-hash = "6e8f0d223b61ab7c100236797f0ec1eed7f23b819c01c117fe445c6b1e91bbfc"
diff --git a/pyproject.toml b/pyproject.toml
index 4ad1c8d4f2..5f7f66e978 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -152,11 +152,18 @@ opentracing = [
     "tornado>=6.0",  # via jaeger-client
 ]
 jwt = ["authlib"]
-# hiredis is not a *strict* dependency, but it makes things much faster.
-# (if it is not installed, we fall back to slow code.)
-# txredisapi 1.4.12 is the first release to accept a `username` kwarg for
-# Redis ACL auth: https://github.com/IlyaSkriblovsky/txredisapi/releases/tag/1.4.12
-redis = ["txredisapi>=1.4.12", "hiredis>=0.3"]
+redis = [
+    # txredisapi 1.4.12 is the first release to accept a `username` kwarg for
+    # Redis ACL auth: https://github.com/IlyaSkriblovsky/txredisapi/releases/tag/1.4.12
+    #
+    # FIXME: 1.6.0 introduces a number of incompatible breaking changes that will
+    # require updates to our own code, see
+    # https://github.com/element-hq/synapse/issues/20347#issuecomment-6154861014
+    "txredisapi>=1.4.12,<1.6.0",
+    # hiredis is not a *strict* dependency, but it makes things much faster.
+    # (if it is not installed, we fall back to slow code.)
+    "hiredis>=0.3"
+]
 # Required to use experimental `caches.track_memory_usage` config option.
 cache-memory = ["pympler>=1.0"]
 # If this is updated, don't forget to update the equivalent lines in
@@ -190,7 +197,7 @@ all = [
     # opentracing
     "jaeger-client>=4.2.0", "opentracing>=2.2.0",
     # redis
-    "txredisapi>=1.4.12", "hiredis>=0.3",
+    "txredisapi>=1.4.12,<1.6.0", "hiredis>=0.3",
     # cache-memory
     # 1.0 added support for python 3.10, our current minimum supported python version
     "pympler>=1.0",