The 27-year-old Suriname international joins the Championship side on loan from Leeds United.
Piroe has excelled in the second division over the past few seasons, scoring 46 goals and providing eight assists in 96 appearances for the Swans.
He then netted 34 times across 118 appearances for Leeds; finishing as the division's top scorer with 19 as the club won the title in 2024/25.
Speaking after joining the Hammers, the former PSV youth product explained that he hopes he can continue to score goals with his new club.
"It's a joy to have got everything over the line. I'm delighted to be here and I'm ready to go," he told his club's website.
"I see West Ham as a Club that really wants to be in the Premier League, and I think should be in the Premier League.
“With my experience, I think I can help the Club a lot to achieve what they want to achieve, and I think it's a good opportunity for me to show what I've learnt and how I've grown as a football player over the years.”
Piroe is West Ham's fifth signing for the new season following the arrivals of Keiber Lamadrid, Joel Veltman, Manor Solomon and Arne Engels.
