diff --git a/docs/changelog.md b/docs/changelog.md
index 1de4790430..3eb4c0b8f0 100644
--- a/docs/changelog.md
+++ b/docs/changelog.md
@@ -4,6 +4,7 @@ Unreleased
 - Added the parameter `matchbox.fhir.context.ssrfProtectionEnabled` to control the new HAPI SSRF protection mechanism
 - Update HAPI FHIR to 8.10.1
 - Fix the GUI not able to send resources larger than 64 KiB (#542)
+- Fix requests larger than 2 MiB being rejected by Undertow (`UT000020`): the maximum request body size is now 100 MB, configurable with `server.undertow.max-http-post-size` (#543)
 
 2026/07/01 Release 4.1.11
 
diff --git a/docs/matchbox-server.md b/docs/matchbox-server.md
index f2cd4eb25d..2be03ddaf5 100644
--- a/docs/matchbox-server.md
+++ b/docs/matchbox-server.md
@@ -75,6 +75,16 @@ hapi:
 Some more configuration parameters used by validation are described in the
 [Validation](validation.md#configuration-parameters) page.
 
+### Maximum request size
+
+Matchbox runs on Undertow, which rejects request bodies larger than 2 MiB by default with
+`RequestTooBigException: UT000020: Connection terminated as request was larger than 2097152`.
+Matchbox raises that limit to 100 MB; if you have to send larger resources, increase
+`server.undertow.max-http-post-size` (e.g. with the environment variable
+`SERVER_UNDERTOW_MAX_HTTP_POST_SIZE`). The value must be positive, Undertow cannot be configured for
+unlimited request bodies through Spring Boot. Note that a reverse proxy in front of Matchbox may
+enforce its own, smaller limit.
+
 ## Read-only mode {: #read-only}
 
 When enabling `httpReadOnly`, the server will reject any operation that would modify its state.
diff --git a/matchbox-server/src/main/resources/application.yaml b/matchbox-server/src/main/resources/application.yaml
index 8d8987f54e..61838f0fd7 100644
--- a/matchbox-server/src/main/resources/application.yaml
+++ b/matchbox-server/src/main/resources/application.yaml
@@ -53,6 +53,12 @@ spring:
 server:
   servlet:
     context-path: /matchboxv3
+  undertow:
+    # Undertow rejects request bodies larger than 2 MiB by default (UndertowOptions.MAX_ENTITY_SIZE),
+    # which is too small for FHIR resources such as documents or large bundles. Override with the
+    # environment variable SERVER_UNDERTOW_MAX_HTTP_POST_SIZE if needed; the value must be positive,
+    # Undertow cannot be configured for unlimited request bodies through Spring Boot.
+    max-http-post-size: 100MB
 management:
   endpoint:
     health: