The 24-year-old former Chelsea and Swansea player was signed by City a year ago but has failed to establish himself, making only three starts in 15 appearances in all competitions last term.
"Scott understands he needs to play football and it will be good to work with him again," West Brom manager Steve Clarke, who knows Sinclair from their Chelsea days, said on the Midlands club's website (www.wba.co.uk).
"We're delighted he feels this is the right club for him at this stage of his career. He is the type of player we were missing from the squad."
West Brom, whose next league match is at Everton on Saturday, said the loan was agreed with a view to a permanent deal. City also confirmed the deal on their website (www.mcfc.co.uk).
(Reporting by Sonia Oxley; Editing by Ken Ferris)
