diff --git a/.changelog/10204.txt b/.changelog/10204.txt
new file mode 100644
index 000000000..0c6f69190
--- /dev/null
+++ b/.changelog/10204.txt
@@ -0,0 +1,3 @@
+```release-note:improvement
+api: The `Content-Type` header is now always set when a body is present in a request.
+```
diff --git a/api/acl.go b/api/acl.go
index d8e0e04d9..2d42f73d8 100644
--- a/api/acl.go
+++ b/api/acl.go
@@ -900,6 +900,7 @@ func (a *ACL) PolicyList(q *QueryOptions) ([]*ACLPolicyListEntry, *QueryMeta, er
 func (a *ACL) RulesTranslate(rules io.Reader) (string, error) {
 	r := a.c.newRequest("POST", "/v1/acl/rules/translate")
 	r.body = rules
+	r.header.Set("Content-Type", "text/plain")
 	rtt, resp, err := requireOK(a.c.doRequest(r))
 	if err != nil {
 		return "", err
diff --git a/api/api.go b/api/api.go
index dbecaa5af..5cdd48680 100644
--- a/api/api.go
+++ b/api/api.go
@@ -871,6 +871,12 @@ func (r *request) toHTTP() (*http.Request, error) {
 	req.Host = r.url.Host
 	req.Header = r.header
 
+	// Content-Type must always be set when a body is present
+	// See https://github.com/hashicorp/consul/issues/10011
+	if req.Body != nil && req.Header.Get("Content-Type") == "" {
+		req.Header.Set("Content-Type", "application/json")
+	}
+
 	// Setup auth
 	if r.config.HttpAuth != nil {
 		req.SetBasicAuth(r.config.HttpAuth.Username, r.config.HttpAuth.Password)
diff --git a/api/api_test.go b/api/api_test.go
index 7b593b22e..56f764d83 100644
--- a/api/api_test.go
+++ b/api/api_test.go
@@ -7,6 +7,7 @@ import (
 	"io/ioutil"
 	"net"
 	"net/http"
+	"net/url"
 	"os"
 	"path/filepath"
 	"reflect"
@@ -810,7 +811,17 @@ func TestAPI_SetWriteOptions(t *testing.T) {
 
 func TestAPI_Headers(t *testing.T) {
 	t.Parallel()
-	c, s := makeClient(t)
+
+	var request *http.Request
+	c, s := makeClientWithConfig(t, func(c *Config) {
+		transport := http.DefaultTransport.(*http.Transport).Clone()
+		transport.Proxy = func(r *http.Request) (*url.URL, error) {
+			// Keep track of the last request sent
+			request = r
+			return nil, nil
+		}
+		c.Transport = transport
+	}, nil)
 	defer s.Stop()
 
 	if len(c.Headers()) != 0 {
@@ -836,6 +847,39 @@ func TestAPI_Headers(t *testing.T) {
 	if r.header.Get("Auth") != "Token" {
 		t.Fatalf("Auth header not set: %v", r.header)
 	}
+
+	kv := c.KV()
+	_, err := kv.Put(&KVPair{Key: "test-headers", Value: []byte("foo")}, nil)
+	require.NoError(t, err)
+	require.Equal(t, "application/octet-stream", request.Header.Get("Content-Type"))
+
+	_, _, err = kv.Get("test-headers", nil)
+	require.NoError(t, err)
+	require.Equal(t, "", request.Header.Get("Content-Type"))
+
+	_, err = kv.Delete("test-headers", nil)
+	require.NoError(t, err)
+	require.Equal(t, "", request.Header.Get("Content-Type"))
+
+	err = c.Snapshot().Restore(nil, strings.NewReader("foo"))
+	require.Error(t, err)
+	require.Equal(t, "application/octet-stream", request.Header.Get("Content-Type"))
+
+	_, err = c.ACL().RulesTranslate(strings.NewReader(`
+	agent "" {
+	  policy = "read"
+	}
+	`))
+	// ACL support is disabled
+	require.Error(t, err)
+	require.Equal(t, "text/plain", request.Header.Get("Content-Type"))
+
+	_, _, err = c.Event().Fire(&UserEvent{
+		Name:    "test",
+		Payload: []byte("foo"),
+	}, nil)
+	require.NoError(t, err)
+	require.Equal(t, "application/octet-stream", request.Header.Get("Content-Type"))
 }
 
 func TestAPI_RequestToHTTP(t *testing.T) {
diff --git a/api/event.go b/api/event.go
index 85b5b069b..1592b7efc 100644
--- a/api/event.go
+++ b/api/event.go
@@ -45,6 +45,7 @@ func (e *Event) Fire(params *UserEvent, q *WriteOptions) (string, *WriteMeta, er
 	if params.Payload != nil {
 		r.body = bytes.NewReader(params.Payload)
 	}
+	r.header.Set("Content-Type", "application/octet-stream")
 
 	rtt, resp, err := requireOK(e.c.doRequest(r))
 	if err != nil {
diff --git a/api/kv.go b/api/kv.go
index 351d287d6..650de0841 100644
--- a/api/kv.go
+++ b/api/kv.go
@@ -205,6 +205,7 @@ func (k *KV) put(key string, params map[string]string, body []byte, q *WriteOpti
 		r.params.Set(param, val)
 	}
 	r.body = bytes.NewReader(body)
+	r.header.Set("Content-Type", "application/octet-stream")
 	rtt, resp, err := requireOK(k.c.doRequest(r))
 	if err != nil {
 		return false, nil, err
diff --git a/api/snapshot.go b/api/snapshot.go
index e902377dd..0c8294f37 100644
--- a/api/snapshot.go
+++ b/api/snapshot.go
@@ -38,6 +38,7 @@ func (s *Snapshot) Save(q *QueryOptions) (io.ReadCloser, *QueryMeta, error) {
 func (s *Snapshot) Restore(q *WriteOptions, in io.Reader) error {
 	r := s.c.newRequest("PUT", "/v1/snapshot")
 	r.body = in
+	r.header.Set("Content-Type", "application/octet-stream")
 	r.setWriteOptions(q)
 	_, _, err := requireOK(s.c.doRequest(r))
 	if err != nil {