Carvalhal has been handed an initial contract until the end of the season that includes the option of an extension and succeeds Paul Clement, who was sacked on 20 December after less than a year in charge with the club sitting at the foot of the table. Player-coach Leon Britton had presided over the 1-1 draw with Crystal Palace and the heavy 5-0 loss at Liverpool.
In a statement, Swansea said that the 52-year-old former Braga, Sporting CP and Besiktas boss agreed to join Swansea late on Wednesday evening (27 December) and travelled to South Wales overnight to meet his new players and take training this morning.
"We are very pleased with the appointment," said chairman Huw Jenkins. "We felt we needed to get the managerial situation resolved as soon as possible and while the timing is not always perfect, we can look forward to the second half of the campaign with renewed optimism."
Carvalhal was named as Stuart Gray's replacement as Wednesday head coach back in June 2015 and led the club to successive play-off appearances, where they were beaten by Hull City and Huddersfield Town in the final and semi-finals respectively.
His standing among Owls supporters deteriorated after a poor first half to the 2017-18 campaign and he left Hillsborough after two-and-a-half years in charge on Christmas Eve following a third consecutive defeat at the hands of Middlesbrough that extended their winless run to seven matches and left them 15th in the table.
'Boro's Garry Monk was also dismissed after that match and quickly installed as the favourite to return to Swansea, the club with whom he previously spent more than 11 years as a player and manager. The likes of Frank de Boer, Slaven Bilic, Slavisa Jokanovic and Ronald Koeman had also been linked with the vacant post.
"Carlos' time at Sheffield Wednesday, which included securing two consecutive play-off places, highlighted that he has great character and personality to lead the squad and the football club, which is vital at such a crucial time," Jenkins added.
"He has built up a lot of experience over the years which will ensure the team is structured and organised. He is driven and ambitious and while he has a big challenge ahead of him, I know he will tackle it without fear."
Carvalhal, whose backroom team will be announced "in due course", takes charge of a struggling Swansea side that are currently two points adrift at the bottom of the Premier League after a Boxing Day thrashing at Anfield and five away from safety.
He will take the reins for the first time at Watford on Saturday before a difficult January stretch that includes matches against Tottenham Hotspur, Liverpool and Arsenal in addition to a crucial clash with Newcastle United and an FA Cup third round tie against second-tier leaders Wolverhampton Wanderers.