The 32-year-old has not played for West Ham since he suffered a freak injury against Swansea in March 2018, when he was knocked unconscious and twisted his knee as he fell awkwardly.
Reid recently returned from a loan spell at Major League Soccer side Sporting Kansas City and has joined Brentford as they chase promotion to the Premier League.
The deal is a straight loan and does not include an option to buy the New Zealand international, whose West Ham contract runs until 2023.
Brentford manager Thomas Frank said: “We are very happy to welcome Winston to Brentford. We are very pleased that he wanted to come to help the team for the rest of the season. With his quality, personality and experience, we know he will do a great job for us over the next few months.
“We felt we needed to bring an extra centre back in to the club. Pontus Jansson will be out for at least six weeks and Luke Racic came back from his loan at Northampton Town with a back injury, so they are not available.
“Our medical staff are also helping Charlie Goode with his ongoing medical issue, so he has not been able to be part of our squad. We were a little short at centre back and Winston will be able to challenge for those places.”
Reid came through the youth ranks at FC Midtjylland, the Danish side where Brentford owner Matthew Benham has the majority shareholding.