Maharaj, South Africa's left-arm spinner, will arrive ahead of Middlesex's third Championship game against Nottinghamshire on April 20 and will be available for eight four-day fixtures in total, as well as the entirety of the T20 Blast.
"We are delighted that Keshav has signed for Middlesex this season and are really excited to have someone of his calibre and experience joining us for the first four months of the season," Coleman said. "The young spinners we have in our squad will benefit enormously from having Keshav with us this year."
Maharaj will be playing for his third different county, after previous stints with Lancashire and Yorkshire. He has taken 72 wickets at 21.72 across his 13 games in the Championship.
"I'm really excited to be linking up with such a professional and experienced county and am looking forward to wearing the Middlesex colours and calling the Home of Cricket my home," he said.