diff --git a/ca/ca.go b/ca/ca.go
index e55b2d66560..509adc972c6 100644
--- a/ca/ca.go
+++ b/ca/ca.go
@@ -126,7 +126,7 @@ type certificateAuthorityImpl struct {
 	issuers      issuerMaps
 	certProfiles certProfilesMaps
 
-	prefix    int // Prepended to the serial number
+	prefix    uint8 // Prepended to the serial number
 	maxNames  int
 	keyPolicy goodkey.KeyPolicy
 	clk       clock.Clock
@@ -233,7 +233,7 @@ func NewCertificateAuthorityImpl(
 	boulderIssuers []*issuance.Issuer,
 	defaultCertProfileName string,
 	certificateProfiles map[string]*issuance.ProfileConfig,
-	serialPrefix int,
+	serialPrefix uint8,
 	maxNames int,
 	keyPolicy goodkey.KeyPolicy,
 	logger blog.Logger,
@@ -244,7 +244,7 @@ func NewCertificateAuthorityImpl(
 	var err error
 
 	if serialPrefix < 1 || serialPrefix > 127 {
-		err = errors.New("serial prefix must be between 1 and 127")
+		err = errors.New("serial prefix must be between 1 (0x01) and 127 (0x7f)")
 		return nil, err
 	}
 
diff --git a/ca/ca_test.go b/ca/ca_test.go
index 3dc12206dea..a8fe83eec71 100644
--- a/ca/ca_test.go
+++ b/ca/ca_test.go
@@ -103,7 +103,7 @@ type testCtx struct {
 	crl                    *crlImpl
 	defaultCertProfileName string
 	certProfiles           map[string]*issuance.ProfileConfig
-	serialPrefix           int
+	serialPrefix           uint8
 	maxNames               int
 	boulderIssuers         []*issuance.Issuer
 	keyPolicy              goodkey.KeyPolicy
@@ -237,7 +237,7 @@ func setup(t *testing.T) *testCtx {
 		crl:                    crl,
 		defaultCertProfileName: "legacy",
 		certProfiles:           certProfiles,
-		serialPrefix:           17,
+		serialPrefix:           0x11,
 		maxNames:               2,
 		boulderIssuers:         boulderIssuers,
 		keyPolicy:              keyPolicy,
@@ -257,7 +257,7 @@ func TestSerialPrefix(t *testing.T) {
 		nil,
 		"",
 		nil,
-		0,
+		0x00,
 		testCtx.maxNames,
 		testCtx.keyPolicy,
 		testCtx.logger,
@@ -271,7 +271,7 @@ func TestSerialPrefix(t *testing.T) {
 		nil,
 		"",
 		nil,
-		128,
+		0x80,
 		testCtx.maxNames,
 		testCtx.keyPolicy,
 		testCtx.logger,
diff --git a/cmd/boulder-ca/main.go b/cmd/boulder-ca/main.go
index b82d76db73e..fe5961c88f6 100644
--- a/cmd/boulder-ca/main.go
+++ b/cmd/boulder-ca/main.go
@@ -5,6 +5,7 @@ import (
 	"flag"
 	"os"
 	"reflect"
+	"strconv"
 	"time"
 
 	"github.com/letsencrypt/boulder/ca"
@@ -61,15 +62,26 @@ type Config struct {
 		}
 
 		// How long issued certificates are valid for.
-		// Deprecated: Use Issuace.CertProfiles.MaxValidityPeriod instead.
+		// Deprecated: Use Issuance.CertProfiles.MaxValidityPeriod instead.
 		Expiry config.Duration
 
 		// How far back certificates should be backdated.
-		// Deprecated: Use Issuace.CertProfiles.MaxValidityBackdate instead.
+		// Deprecated: Use Issuance.CertProfiles.MaxValidityBackdate instead.
 		Backdate config.Duration
 
 		// What digits we should prepend to serials after randomly generating them.
-		SerialPrefix int `validate:"required,min=1,max=127"`
+		// Deprecated: Use SerialPrefixHex instead.
+		SerialPrefix int `validate:"required_without=SerialPrefixHex,omitempty,min=1,max=127"`
+
+		// What hex string we should prepend to serials after randomly
+		// generating them. The minimum value is "01" to ensure that at least
+		// one bit in the prefix byte is set. The maximum value is "7f" to
+		// ensure that the first bit in the prefix byte is not set. The validate
+		// library cannot enforce mix/max values on strings, so that is done in
+		// NewCertificateAuthorityImpl.
+		//
+		// TODO(#7213): Replace `required_without` with `required` when SerialPrefix is removed.
+		SerialPrefixHex string `validate:"required_without=SerialPrefix,omitempty,hexadecimal,len=2"`
 
 		// MaxNames is the maximum number of subjectAltNames in a single cert.
 		// The value supplied MUST be greater than 0 and no more than 100. These
@@ -152,6 +164,13 @@ func main() {
 		c.CA.DebugAddr = *debugAddr
 	}
 
+	serialPrefix := uint8(c.CA.SerialPrefix)
+	if c.CA.SerialPrefixHex != "" {
+		parsedSerialPrefix, err := strconv.ParseUint(c.CA.SerialPrefixHex, 16, 8)
+		cmd.FailOnError(err, "Couldn't convert SerialPrefixHex to int")
+		serialPrefix = uint8(parsedSerialPrefix)
+	}
+
 	if c.CA.MaxNames == 0 {
 		cmd.Fail("Error in CA config: MaxNames must not be 0")
 	}
@@ -280,7 +299,7 @@ func main() {
 			issuers,
 			c.CA.Issuance.DefaultCertificateProfileName,
 			c.CA.Issuance.CertProfiles,
-			c.CA.SerialPrefix,
+			serialPrefix,
 			c.CA.MaxNames,
 			kp,
 			logger,
diff --git a/test/config-next/ca.json b/test/config-next/ca.json
index 9c5fa449f63..346ccc63c1b 100644
--- a/test/config-next/ca.json
+++ b/test/config-next/ca.json
@@ -142,7 +142,7 @@
 				}
 			]
 		},
-		"serialPrefix": 127,
+		"serialPrefixHex": "7f",
 		"maxNames": 100,
 		"lifespanOCSP": "96h",
 		"goodkey": {