The 22-year-old moves to the Philips Stadion after so far failing to break into the first team at the Stade Louis II.
Former France Under-21 international Isimat-Mirin joined Monaco in July last year from Valenciennes.
However, the former Rennes man made just 11 appearances in all competitions in his maiden season with the Principality club as Monaco finished second in Ligue 1 behind champions Paris Saint-Germain.
He will now hope for better fortune with a PSV side that has made a fantastic start to the season, winning all four of their opening Eredivisie matches.
Isimat-Mirin could make his PSV debut when the Eindhoven club take on PEC Zwolle a week on Saturday.