The Seattle Mariners drafted Tuiasosopo 93rd overall in the third round in the 2004 Major League Baseball Draft.
He began his pro career with the Peoria Mariners in 2004, hitting a solo home run in first career-at bat on July 10.
Tuiasosopo hit safely in 18 of 20 games and reached base safely in 19 of 20 games.
He had a 13-game hit streak, hitting .467 from July 10–25, and tied for the team lead in home runs with four.
On August 5, he was promoted to the Everett AquaSox.
At the end of the season, he participated in the 2004 Arizona Instructional League.

In 2005, he was with Seattle's Low-A affiliate, the Wisconsin Timber Rattlers.
He hit .276 with three home runs and 45 runs batted in.
He was ranked number 36 on Minor League News' annual Fab 50.
He recorded a season-high 13-game hitting streak to start the season, batting .423 with eight runs, three doubles, two home runs and 12 RBIs during that time.
Tuiasosopo had 27 multi-hit games, three four-hit games and six three-hit games.
He hit home runs in back-to-back games on June 9 and 10.

In 2006, he split time between the Mariners' High-A Inland Empire 66ers and Double-A San Antonio Missions affiliates.
He batted a combined .248 with two home runs and 44 RBI.
He played with the Peoria Javelinas of the Arizona Fall League that fall.
He hit .306 with one home run with 34 RBIs, 31 runs scored and five stolen bases in 59 games with Inland Empire but only batted .185 with one home run, 16 runs scored and 10 RBIs with San Antonio.

Tuiasosopo spent the 2007 season with the Double-A West Tenn Diamond Jaxx.
He hit .424 with eight runs, five doubles and three RBIs during a season-high 10-game hitting streak from April 5–14.
He recorded a double in five consecutive games from April 6-1.
He was named to the Southern League All-Star team and later that season played for the Peoria Javelinas in the Arizona Fall League.

At the end of the 2007 campaign Tuiasosopo was rated as the Mariners No.
10 prospect by Baseball America.
He was named to the Minor League News Fab 50 again in 2007.
The Seattle Mariners drafted Tuiasosopo 93rd overall in the third round in the 2004 Major League Baseball Draft. He began his pro career with the Peoria Mariners in 2004, hitting a solo home run in first career-at bat on July 10. Tuiasosopo hit safely in 18 of 20 games and reached base safely in 19 of 20 games. He had a 13-game hit streak, hitting .467 from July 10–25, and tied for the team lead in home runs with four. On August 5, he was promoted to the Everett AquaSox. At the end of the season, he participated in the 2004 Arizona Instructional League.
In 2005, he was with Seattle's Low-A affiliate, the Wisconsin Timber Rattlers. He hit .276 with three home runs and 45 runs batted in. He was ranked number 36 on Minor League News' annual Fab 50. He recorded a season-high 13-game hitting streak to start the season, batting .423 with eight runs, three doubles, two home runs and 12 RBIs during that time. Tuiasosopo had 27 multi-hit games, three four-hit games and six three-hit games. He hit home runs in back-to-back games on June 9 and 10.
In 2006, he split time between the Mariners' High-A Inland Empire 66ers and Double-A San Antonio Missions affiliates. He batted a combined .248 with two home runs and 44 RBI. He played with the Peoria Javelinas of the Arizona Fall League that fall. He hit .306 with one home run with 34 RBIs, 31 runs scored and five stolen bases in 59 games with Inland Empire but only batted .185 with one home run, 16 runs scored and 10 RBIs with San Antonio.
Tuiasosopo spent the 2007 season with the Double-A West Tenn Diamond Jaxx. He hit .424 with eight runs, five doubles and three RBIs during a season-high 10-game hitting streak from April 5–14. He recorded a double in five consecutive games from April 6-1. He was named to the Southern League All-Star team and later that season played for the Peoria Javelinas in the Arizona Fall League.
At the end of the 2007 campaign Tuiasosopo was rated as the Mariners No. 10 prospect by Baseball America. He was named to the Minor League News Fab 50 again in 2007.
Matt Tuiasosopo