The Portuguese forward, who spent nine years at Real Madrid and is the club's all-time top goalscorer, invested in Almeria through his company CR7 Sports.
Almeria, who were relegated from LaLiga in 2023.24 and were taken over by Saudi-based SMC Group in 2025, did not disclose the financial terms of the deal.
"The agreement is part of the international expansion of the club driven by president Mohamed Al Kereiji through his business holding SMC Group," Almeria said in a statement.
Ronaldo, 41, has been playing for Saudi Arabian top-flight club Al Nassr since 2023.
"UD Almeria is a Spanish club with a strong foundation and clear growth potential. I wish to work with the team leading the club to support it in its new phase of growth," Ronaldo said in a statement shared by Almeria.
Almeria, who have one second-tier title in their trophy cabinet, are currently third in the Spanish second division table.