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Seattle Sounders FC announced the creation of their reserve team on October 14, 2014, at an event held at the Chihuly Garden and Glass museum in Seattle. The new team would be named Seattle Sounders FC 2 (shortened to S2) and play in USL Pro, the second tier of American soccer that the Sounders played in until 2009. S2's home matches were to be played at the MLS team's training facility, Starfire Sports Complex in Tukwila, beginning in the 2015 season. The Sounders intended to primarily use the team to develop young players and split ownership to a non-profit, fan-owned organization called the Sounders Community Trust. A similar reserve team for the Portland Timbers, T2, was also announced on the same day. Sounders assistant coach Ezra Hendrickson was named the team's coach on November 13, 2014.

Inaugural season

S2 played its inaugural match on March 21, 2015, winning 4–2 over the defending USL champion Sacramento Republic. Andy Craven notched the first goal in team history. Their second game yielded the team's first shutout, a home game against Whitecaps FC 2 that ended 4–0. Darwin Jones scored the team's first hat-trick during the game.

Tacoma relocation

The owners of the Tacoma Rainiers baseball team had expressed interest in hosting a lower-division soccer team at its ballpark, Cheney Stadium, or a separate soccer stadium as early as 2013. Cheney Field hosted an MLS Reserve League match between the Sounders and Orlando City SC reserve teams in May 2013, which brought an attendance of 2,174. The Rainiers partnered with the Tacoma Stars, an indoor soccer team, and discussed a potential move for S2 with the Sounders organization. The Sounders had previously discussed plans to move their lower-league franchise to the Tacoma area in the 2000s in the event of a successful MLS expansion bid.

On May 6, 2017, the Sounders and Rainiers announced that they had agreed to relocate the reserve team to Tacoma upon completion of a new, 5,000-seat soccer-specific stadium by 2020. After the USL announced its intention to refuse waivers for teams with smaller venues that did not meet full Division II requirements, the Sounders announced in November 2017 that S2 would temporarily play at Cheney Stadium in Tacoma beginning in the 2018 season. The team had previously played in front of crowds of less than 1,000 at Starfire Sports, but debuted at Cheney Stadium with a sellout crowd of 6,049 on March 16, 2018. The team failed to qualify for the USL playoffs, but the first season in Tacoma garnered an average attendance of over 3,000.

The club was re-branded as the Tacoma Defiance on January 30, 2019, following a campaign to solicit suggestions from the public for a new name. The Defiance name was the top choice in the poll and references Point Defiance Park and Tacoma's civic pride. MultiCare Health System was named as the team's jersey sponsor and Reign FC of the National Women's Soccer League also announced a move to Cheney Stadium for the 2019 season. Sounders FC Academy Director Chris Little had been named as the team's new head coach a week earlier, replacing John Hutchinson.

MLS Next Pro

The club announced on December 6, 2021, that it was joining the inaugural 21-team MLS Next Pro season starting in 2022.