The Costa Rica international has returned to the country where he made his name as he looks for regular first-team football ahead of next summer's World Cup
Ruiz has struggled for form and fitness this season, making just 14 appearances for the Cottagers, and returns to the Netherlands in search of regular football ahead of next summer's World Cup.
The Costa Rica international arrived at Craven Cottage from FC Twente in the summer of 2011 but has failed to reproduce the prolific scoring form which characterised his time in the Eredivisie.
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I've got a lot of good memories of my time at FC Twente, but I'm a professional and will give 100 per cent to reach our goals with PSV.
As soon as I had the chance to go to PSV, I was interested right away. I'm looking forward to starting training. I'm very motivated to play for PSV.
"It's correct that I didn't play that much at Fulham. That's why I see this as a great oppurtunity.
"I want to get my trust back, help PSV to reach our goals, finish as high as possible and prepare for the World Cup.
"PSV have got a lot of quality and we'll try our best to return to the highest places in all the competitions."
Despite producing moments of brilliance under Martin Jol, Ruiz became a target for Fulham fans angry at his perceived lack of effort as the club slipped into relegation trouble and the Dutchman lost his job.
PSV, currently managed by former Netherlands international and Barcelona star Phillip Cocu, currently lie seventh in the Eredivisie.