The following are known Action Replay Codes for Ready 2 Rumble Boxing: Round 2 on Nintendo Game Boy Advance (GBA).
These codes were also known as GameShark Codes and in most cases will work on both devices.

These codes are for the U.S. version of the game.

Master Code – Must Be On
308fd31c
f98e5ff9
2ce5d101
b14db71c

Inf Health
fac88a9e
d04168ca
b14b6d72
1c1d266c

Inf Stamina
032d4c35
fbad7c49
e17e80bb
9940507a

Max Money
91dcd56b
62090662

Unlock Rumble Man
439e15b1
9da667ba

Unlock Shaq
0a604df1
7f346745

Unlock M. Jackson
6da7bb17
1dd9d139

Freeze Round Timer
744be0b4
ccbe0e2e

1 Hit To KO P2
cd1482c0
215b5d22
a95d9970
8e4be69b

Always Max Rumble
5e144f15
59d03a2a

P1 No Rumble
4b68fa8d
548e6954

Opponent Rumble On
1b6f3a53
fa684d72

Opponent No Rumble
7e5d7d51
80a165da

Max Weight Lifting
7d4bd604
3b123a88

Max Heavy Bag
0fbb9a5c
f9f94bb8

Max Speed Bag
eb6b583a
23e8e50f

Max Rumble Pads
dc1cf62c
e05517f8
08d8f7c4
74a7d208
eecb8436
e989e835

Max STR
e02c3f17
24aea898

Max STM
b1cec889
4787655d

Max SPD
fb6df76f
2e539de3

Always January
fc7472c1
93fd8e63

Press AR Button: Slow
dbba3a89
4cb74b94