Seattle Mariners On 20 November 2010, Vieira signed with the
Seattle Mariners organization as an international
free agent for $65,000. At the time of his signing, he was the second Brazilian-born pitcher to reach the Major Leagues. The first Brazilian pitcher to reach MLB was
André Rienzo in Chicago White Sox (2006). Thyago made his professional debut with the
Venezuelan Summer League (VSL) Mariners, pitching to a 4.00
earned run average (ERA) in 12 games. He returned to the VSL Mariners the next year, logging a 3–5
win–loss record and 6.05 ERA in 55 innings of work. In 2013, Vieira played for the Low-A
Everett AquaSox, pitching to a 4–5 record and 3.84 ERA with 51 strikeouts in 68 innings of work. In 2014, Vieira played for the Single-A
Clinton LumberKings, recording a 1–1 record and 5.23 ERA in 13 games. He returned to Clinton for the 2015 season, and accrued a 1–4 record and 6.97 ERA in 22 appearances for the team. The Mariners added Vieira to their
40-man roster after the 2016 season. He began the 2017 season with the Double-A
Arkansas Travelers, and also played for the Triple-A
Tacoma Rainiers, logging a 4.00 ERA in 41 appearances between the two clubs. Vieira was promoted to the major leagues for the first time on 14 August 2017, and pitched one shutout inning against the
Baltimore Orioles before being optioned to Triple-A the next day.
Chicago White Sox On 16 November 2017, Vieira was traded to the
Chicago White Sox in exchange for international bonus pool money. He was assigned to the Triple-A
Charlotte Knights to begin the 2018 season, where he logged a 5.05 ERA in 36 games. In 16 games for Chicago, Vieira pitched to a 7.13 ERA with 15 strikeouts in 16 appearances. In 2019, he struggled to a 9.00 ERA in six big league games, but recorded a better 6–4 record and 5.70 ERA in 39 games for Triple-A Charlotte. Vieira was released on 2 December 2019, in order to allow him to pursue an opportunity in Japan.
Yomiuri Giants On 3 December 2019, Vieira signed with the
Yomiuri Giants of
Nippon Professional Baseball (NPB). In his first NPB season, Vieira had a 3.28 ERA with 29 strikeouts in innings of work. On 13 August 2021, Vieira recorded the fastest pitch in NPB history at , a record previously held by
Shohei Ohtani and
Robert Corniel. On 1 September 2021, he set a NPB record for most consecutive mound appearances without giving up a run (32) by a non-Japanese player, breaking a record previously set in 2011 by
Brian Falkenborg of the
Fukuoka SoftBank Hawks. In 2021, he recorded seven saves and had a 2.76 ERA in innings with 38 strikeouts and was selected for the Nippon League All Star Game. On 23 December 2021, Vieira re-signed with the Giants for the 2022 season. On 7 April 2022, following a game in which he had hit
Hiroshima Toyo Carp batter
Ryan McBroom in the head, Vieira was sent down to Yomiuri's farm team. He had struggled to a 22.50 ERA in 4 appearances at the point of his demotion.
Milwaukee Brewers On 12 January 2023, Vieira signed a minor league contract with the
Milwaukee Brewers organization. In 33 games for the Triple–A
Nashville Sounds, he posted a 3.35 ERA with 51 strikeouts and 8
saves in innings pitched. On 6 September 2023, Vieira's contract was selected by the Brewers, adding him to the major league roster. He made two scoreless appearances for Milwaukee before the conclusion of the season, compiling two strikeouts in three innings pitched. Vieira made 16 appearances out of the
bullpen for Milwaukee in 2024, but struggled to a 5.64 ERA with 25 strikeouts across innings of work. He was
designated for assignment by the Brewers on 20 May 2024.
Baltimore Orioles On 25 May 2024, Vieira and Aneuris Rodriguez were traded to the
Baltimore Orioles in exchange for Garrett Stallings. Vieira allowed all five batters he faced to reach base on four
walks and a three-run
triple in the eighth inning of an 11–3
home win over the
Boston Red Sox which was his Orioles debut two days later on 27 May. He was designated for assignment following the promotion of
Connor Norby on 3 June.
Arizona Diamondbacks On 6 June 2024, Vieira was claimed off waivers by the
Arizona Diamondbacks. In 11 appearances for Arizona, he recorded a 2.87 ERA with 14 strikeouts across innings pitched. Vieira was designated for assignment by the Diamondbacks on 27 July. He cleared waivers and was sent outright to the Triple–A
Reno Aces on 2 August. On 27 February 2025, it was announced that Vieira would require
Tommy John surgery and miss the entirety of the 2025 season. He was released by the Diamondbacks on 27 March.
Toros de Tijuana On 16 December 2025, Vieira signed with the
Toros de Tijuana of the
Mexican League. ==International career==