The station, on the Belair line above the Goodwood Rd underpass, was closed in 1995 because of poor patronage.
Mr Koutsantonis wrote to a resident in May ruling out reopening the station.
But a Transport Department spokeswoman said he had changed his mind following the renewed push to have it reopened.
"Responding to representations from members of the local community, Member for Ashford Steph Key and the Unley Council, the Minister … instructed the department to initiate a detailed investigation of the possibility of reopening Millswood Station," she said.
She could not provide details of the investigation, including what it would take to have the station reopened, the timeframe or cost of the investigation.
There have been multiple attempts by residents to have Millswood Station reopened, most recently in 2009 when about 100 people attended a meeting at the station.
Millswood resident John Gasper, part of the group pushing for the reopening, welcomed the minister's decision to revisit the issue.
Unley state Liberal MP David Pisoni, who told the Eastern Courier Messenger about May letter, said the investigation was only launched because there was an election looming.
"This announcement should be taken with a pinch of salt because they've already ruled it out," Mr Pisoni said.
"I believe that, with the fact we now have an upgraded track, the closure should be revisited."