diff --git a/README.md b/README.md
index b13178e..6545478 100644
--- a/README.md
+++ b/README.md
@@ -96,6 +96,7 @@ environment variables that you can set.
 | `EAB_HMAC_KEY`              | The Base64-encoded EAB HMAC key to use when provisioning TLS certificates, if required. | None |
 | `FORWARD_HEADERS`           | Whether to forward X-Forwarded-* and X-Request-ID headers from the client. | Disabled when running with TLS; enabled otherwise |
 | `LOG_REQUESTS`              | Log all requests. Set to `0` or `false` to disable request logging | Enabled |
+| `ENCODE_QUERY_SEMICOLONS`   | Set to `1` or `true` to encode raw `;` in query strings as `%3B`, so they're passed upstream as part of the value. Otherwise Go drops any query parameter containing a raw `;`. | Disabled |
 | `DEBUG`                     | Set to `1` or `true` to enable debug logging. | Disabled |
 
 To prevent naming clashes with your application's own environment variables,
diff --git a/internal/config.go b/internal/config.go
index fd2887d..f890f0b 100644
--- a/internal/config.go
+++ b/internal/config.go
@@ -72,6 +72,8 @@ type Config struct {
 
 	ForwardHeaders bool
 
+	EncodeQuerySemicolons bool
+
 	LogLevel    slog.Level
 	LogRequests bool
 }
@@ -114,6 +116,8 @@ func NewConfig() (*Config, error) {
 
 		H2CEnabled: getEnvBool("H2C_ENABLED", defaultH2CEnabled),
 
+		EncodeQuerySemicolons: getEnvBool("ENCODE_QUERY_SEMICOLONS", false),
+
 		LogLevel:    logLevel,
 		LogRequests: getEnvBool("LOG_REQUESTS", defaultLogRequests),
 	}
diff --git a/internal/config_test.go b/internal/config_test.go
index 5dc1d9b..48ab7a2 100644
--- a/internal/config_test.go
+++ b/internal/config_test.go
@@ -106,6 +106,7 @@ func TestConfig_defaults(t *testing.T) {
 	assert.Equal(t, defaultCacheSize, c.CacheSizeBytes)
 	assert.Equal(t, slog.LevelInfo, c.LogLevel)
 	assert.Equal(t, false, c.H2CEnabled)
+	assert.Equal(t, false, c.EncodeQuerySemicolons)
 }
 
 func TestConfig_override_defaults_with_env_vars(t *testing.T) {
@@ -121,6 +122,7 @@ func TestConfig_override_defaults_with_env_vars(t *testing.T) {
 	usingEnvVar(t, "H2C_ENABLED", "true")
 	usingEnvVar(t, "GZIP_COMPRESSION_DISABLE_ON_AUTH", "true")
 	usingEnvVar(t, "GZIP_COMPRESSION_JITTER", "64")
+	usingEnvVar(t, "ENCODE_QUERY_SEMICOLONS", "true")
 
 	c, err := NewConfig()
 	require.NoError(t, err)
@@ -136,6 +138,7 @@ func TestConfig_override_defaults_with_env_vars(t *testing.T) {
 	assert.Equal(t, true, c.H2CEnabled)
 	assert.Equal(t, true, c.GzipCompressionDisableOnAuth)
 	assert.Equal(t, 64, c.GzipCompressionJitter)
+	assert.Equal(t, true, c.EncodeQuerySemicolons)
 }
 
 func TestConfig_override_defaults_with_env_vars_using_prefix(t *testing.T) {
diff --git a/internal/handler.go b/internal/handler.go
index 61086dc..ae16e37 100644
--- a/internal/handler.go
+++ b/internal/handler.go
@@ -18,11 +18,13 @@ type HandlerOptions struct {
 	gzipCompressionJitter        int
 	forwardHeaders               bool
 	logRequests                  bool
+	encodeQuerySemicolons        bool
 }
 
 func NewHandler(options HandlerOptions) http.Handler {
 	handler := NewProxyHandler(options.targetUrl, options.badGatewayPage, options.forwardHeaders)
 	handler = NewCacheHandler(options.cache, options.maxCacheableResponseBody, handler)
+	handler = NewQuerySemicolonHandler(options.encodeQuerySemicolons, handler)
 	handler = NewSendfileHandler(options.xSendfileEnabled, handler)
 	handler = NewRequestStartHandler(handler)
 	handler = NewRequestIDHandler(options.forwardHeaders, handler)
diff --git a/internal/query_semicolon_handler.go b/internal/query_semicolon_handler.go
new file mode 100644
index 0000000..b51da09
--- /dev/null
+++ b/internal/query_semicolon_handler.go
@@ -0,0 +1,28 @@
+package internal
+
+import (
+	"net/http"
+	"strings"
+)
+
+// Go drops query pairs containing a raw ";", so encode it to keep it as part of the value.
+func NewQuerySemicolonHandler(enabled bool, next http.Handler) http.Handler {
+	if !enabled {
+		return next
+	}
+
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if !strings.Contains(r.URL.RawQuery, ";") {
+			next.ServeHTTP(w, r)
+			return
+		}
+
+		r2 := new(http.Request)
+		*r2 = *r
+		u := *r.URL
+		u.RawQuery = strings.ReplaceAll(r.URL.RawQuery, ";", "%3B")
+		r2.URL = &u
+
+		next.ServeHTTP(w, r2)
+	})
+}
diff --git a/internal/query_semicolon_handler_test.go b/internal/query_semicolon_handler_test.go
new file mode 100644
index 0000000..b08a954
--- /dev/null
+++ b/internal/query_semicolon_handler_test.go
@@ -0,0 +1,86 @@
+package internal
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestQuerySemicolonHandler_encodes_semicolons_when_enabled(t *testing.T) {
+	tests := map[string]string{
+		"/?a=1;2&b=3":   "a=1%3B2&b=3",
+		"/?a=1%3B2&b=3": "a=1%3B2&b=3",
+		"/?a=1%3b2&b=3": "a=1%3b2&b=3",
+		"/?a=1;;2":      "a=1%3B%3B2",
+	}
+
+	for target, expected := range tests {
+		t.Run(target, func(t *testing.T) {
+			var rawQuery, value string
+			upstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				rawQuery = r.URL.RawQuery
+				value = r.URL.Query().Get("a")
+			}))
+			defer upstream.Close()
+
+			options := handlerOptions(upstream.URL)
+			options.encodeQuerySemicolons = true
+			h := NewHandler(options)
+
+			w := httptest.NewRecorder()
+			h.ServeHTTP(w, httptest.NewRequest("GET", target, nil))
+
+			assert.Equal(t, http.StatusOK, w.Code)
+			assert.Equal(t, expected, rawQuery)
+			assert.Contains(t, value, ";")
+		})
+	}
+}
+
+func TestQuerySemicolonHandler_drops_semicolon_pairs_when_disabled(t *testing.T) {
+	var rawQuery string
+	upstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		rawQuery = r.URL.RawQuery
+	}))
+	defer upstream.Close()
+
+	h := NewHandler(handlerOptions(upstream.URL))
+
+	w := httptest.NewRecorder()
+	h.ServeHTTP(w, httptest.NewRequest("GET", "/?a=1;2&b=3", nil))
+
+	assert.Equal(t, http.StatusOK, w.Code)
+	assert.Equal(t, "b=3", rawQuery)
+}
+
+func TestQuerySemicolonHandler_keeps_cache_keys_distinct(t *testing.T) {
+	upstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Cache-Control", "public, max-age=60")
+		_, _ = w.Write([]byte(r.URL.Query().Get("a")))
+	}))
+	defer upstream.Close()
+
+	options := handlerOptions(upstream.URL)
+	options.encodeQuerySemicolons = true
+	h := NewHandler(options)
+
+	serveRequest := func(target string) (string, string) {
+		w := httptest.NewRecorder()
+		h.ServeHTTP(w, httptest.NewRequest("GET", target, nil))
+		return w.Header().Get("X-Cache"), w.Body.String()
+	}
+
+	cache, body := serveRequest("/?a=1;2")
+	assert.Equal(t, "miss", cache)
+	assert.Equal(t, "1;2", body)
+
+	cache, body = serveRequest("/")
+	assert.Equal(t, "miss", cache)
+	assert.Equal(t, "", body)
+
+	cache, body = serveRequest("/?a=1%3B2")
+	assert.Equal(t, "hit", cache)
+	assert.Equal(t, "1;2", body)
+}
diff --git a/internal/service.go b/internal/service.go
index c34ba73..58e3e3d 100644
--- a/internal/service.go
+++ b/internal/service.go
@@ -30,6 +30,7 @@ func (s *Service) Run() int {
 		logRequests:                  s.config.LogRequests,
 		gzipCompressionDisableOnAuth: s.config.GzipCompressionDisableOnAuth,
 		gzipCompressionJitter:        s.config.GzipCompressionJitter,
+		encodeQuerySemicolons:        s.config.EncodeQuerySemicolons,
 	}
 
 	handler := NewHandler(handlerOptions)