Liberal Democrat AM Eluned Parrott said the Welsh government had "wasted" more than £52,000 on them at railway stations in south Wales.
She accused ministers of spending public money to "promote themselves before an election".
The Welsh government said it was unapologetic about the campaign.
A spokesman said: "We are not going to apologise for informing the Welsh public about one of the most important economic and social projects in Wales' history.
"Are the Liberal Democrats really saying that we should not be providing people in Wales with this important information?"
Ms Parrott said her party backed the project, but added: "To all intents and purposes, here we have Labour ministers spending over £50,000 of public money to promote themselves before an election.
"This is money spent only on marketing the fact that one day, years in the future, there will be a Metro in south Wales.
"This is a cynical way of spending public money. The minister is wasting tens of thousands of pounds on a useless poster campaign."
Work on the Metro is set to start in 2017 and finish in 2020, a government spokesman said.
Formal consultation for the £600m programme to develop an integrated network of rail, bus and light rail services begins in 2016.
An advertising campaign promoting the South Wales Metro has been criticised as "useless" and a "cynical way of spending public money".