The midfielder started his career at Leeds United, where he played in both the Championship and Premier League before breaking into the England squad and securing a move to Manchester City.
Capped 31 times by England, Phillips started every game as the Three Lions reached the final of UEFA Euro 2020 and appeared at the 2022 FIFA World Cup finals.
His time at City didn't quite go to plan, but Phillips is set for a fresh start in London with a West Ham side chasing European football.
Speaking after his arrival, the former Leeds player said: “I’m really pleased to be here.
“There’s always been a lot of speculation about me moving to West Ham, so I’m really excited to get going now.
“There’s so much to look forward to in the second half of the season for the Club and I can’t wait to be part of it.
“There’s an unbelievable group here and if I can come in and add the quality that I know I’m capable of, I’m sure we can give the West Ham fans plenty to be excited about between now and the end of the season."
The midfielder joins the Hammers and is ready to slot straight into their side, with a potential debut coming against Bournemouth on Thursday, February 1st.