The move opens the door for high-profile Perth Lord Mayor Basil Zempilas, who narrowly secured the seat of Churchlands at the recent election, to seek the leadership now a change is no longer hypothetical.
Ms Mettam said she would seek to stay on as deputy leader.
"While I would have liked and hoped to continue as leader it has become clear that I do not have that support of my colleagues to continue as leader through to the election in 2029," she said.
"I will be the only experienced Liberal in the Legislative Assembly and as a result many colleagues have encouraged me to support our new members and our new leader in the most effective way I can," she said in a statement on Thursday.
The WA Liberals party room will meet on Tuesday but Ms Mettam said it was appropriate to announce her intentions ahead of time.
"A decision once made should not remain secret," she said.
Ms Mettam took responsibility for the party's defeat at the state election earlier in March, but said leadership speculation was a significant contributor to the outcome.
After Labor achieved another resounding win at the March 8 election, Ms Mettam said she would leave the leadership to the party room to decide, as Mr Zempilas was being widely touted as her successor.
Mr Zempilas, a high-profile media personality before becoming Perth mayor, was asked if he would put his hand up for the leadership if Ms Mettam resigned during a debate broadcast on ABC Perth prior to the election.
"That's a hypothetical," he said.
The Liberals gained four seats at the election, including Churchlands.
The party had been in the political wilderness since 2021 when they were left with only two seats following a landslide Labor victory.
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