The 24-year-old forward from Mississauga, Ont., spent last season with the MLS team USL club, the Real Monarchs SLC. Welshman is the first player to sign with RSL from the Monarchs.
Welshman, who played club soccer for Sigma FC growing up in the Toronto area, scored six goals and added two assists in 19 appearances with the Monarchs.
"Emery deserves a lot of credit as he did everything asked of him over the past year. He matured and accepted the challenges given to him by the staff and in the end persevered to end the season on a very strong run," Real Salt Lake general manager Craig Waibel said in a statement.
Welshman was selected 16th overall by Toronto in the 2013 MLS SuperDraft but saw just five minutes of action in league play that season. He spent 2014 playing for Sigma in League 1 before joining the Monarchs.
The former Oregon State forward made his international debut for Guyana last year, scoring three goals in five matches.