Production of its Combo vans will be moved from its plant at Azambuja, 30 miles north of Lisbon, to a larger facility at Zaragoza in Spain.
GM said the cost of producing vehicles made the site uncompetitive, leaving ‘no alternative’ other than closure.
The company will reimburse any financial aid it had received from the government which was ‘undeserved’.
Workers staged a one day strike at the Azambuja plant last month in protest of a possible closure.
As well as its 1,200 production staff, GM employs a further 800 people in Portugal in administrative roles. It is not clear whether their jobs are under threat.