The 23-year-old former Hibernian midfielder admits he is delighted to secure a deal, with Thistle on a high at the moment.
Alan Archibald's men have made a promising start to the campaign on their return to the top flight, with four points from their opening two matches.
"I don't think we could have asked for a better start to the season and the great thing is that the players are really enjoying it," said Welsh.
"The gaffer has done a great job getting so many of last year's squad signed up on lengthy deals and I am glad to have sorted out one for myself.
"He has also added a lot of quality among the new faces so we know that we have to play well to keep our places in the team. It is early days but we are pleased with the start that we have made."