Baharudin, 23, has 43 international caps and attended City's training camp in Abu Dhabi from 10-19 January as part of a development opportunity arranged with the Singaporean Football Association.
A versatile defender, the Singaporean impressed as centre-back and fullback during the club's two trial matches in the UAE, earning himself a contract.
Baharudin started against Al Jazira at the Mohammed Bin Zayed Stadium where he performed well against Montenegro and former Juventus striker Mirko Vucinic.
Baharudin also featured as a substitute in the side's 1-1 draw with Ukrainian Premier League outfit Dnipro Dnipropetrovsk at New York University's Abu Dhabi Campus.
City coach John van't Schip said Baharudin would provide his squad with further depth in defence in the second half of the season.
"Safuwan is a mobile defender who is calm and composed on the ball and makes quick and good decisions," Van't Schip said.
"He impressed the coaching staff while playing and training with us in Abu Dhabi and we believe he will fit in with our players and complement our style."
Just two days ago Baharudin was dismissive of transfer rumours linking him with a move away from LionsXII.
Please know that all that's been said about the move are rumours. The reason i'm not with the team in Maldives is due to injury recovery. It has been awhile since I last trained with the team thus the previous post regarding me missing the boys.
However, Baharudin said on Thursday he was delighted to join City for the rest of the 2014-2015 campaign.
"When this opportunity came, I told myself that I had to go all out because this is a once-in-a-lifetime opportunity that may never come again," Baharudin said.
"I worked very hard and I really enjoyed training with the team in Abu Dhabi and got along well with the other players. I am very happy that the coaches were happy with what they saw.
"I am looking forward to doing my best for the club and as an ambassador of Singapore. I want to make my parents and family proud of me."