The following are known Action Replay Codes for Tony Hawk’s Underground 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
8d587104
e561c610
a9b61b83
9fd6f1e4

Infinite Health
a62d0cba
a2c16396

Max Overall Score
45ae0c67
2a852288
0fb35db8
0dcbf2c2

Max Trick Score
fa08fa8e
b635f7fa
b849669e
8c310681

Infinite Cash
24687d30
5f51b5fa

Max Stats
2744a515
2b439178
c1ef1a17
626b0d6a
b59cf0e6
44493959
2e931623
1fd57c98
c70d9d1f
8ea80ea4
31c130a1
1c5248d3
c2c09d44
2e30a907
da768881
584cf032