The Philippines-based operator had itself bid for the company, but withdrew after Mitsui entered a higher bid.
Portek will now be de-listed from the Singapore stock exchange, which it first entered in 2002.
ICTSIs Portek share bought out by Mitsui
ICTSI has received $30m for its 16.9% stake in Portek International from new majority owner Mitsui & Co (97.04%).