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 assume the leadership now he has been elected to parliament and a change is no longer hypothetical.
Ms Mettam said she would step forward as a candidate for the role of deputy.
"While I would have liked and hoped to continue as leader it has become clear 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.
She vowed to support the new leader, whoever it ends up being, regardless of whether she secures the deputy's job.
"I understand through conversations that there is support for Basil Zempilas going forward as the leader but we need to give an opportunity for our incoming candidates to ultimately make that decision," she told reporters.
"I'm not aware of anyone else putting up their hand," she added.
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 if Ms Mettam resigned during a debate broadcast on ABC Perth prior to the election.
"That's a hypothetical," he said.
Leadership instability heightened in November after leaked polling suggested the party's vote could be higher at the election if Mr Zempilas led the campaign.
"You can only take people at their word and Basil has been very explicit and clear, publicly and privately, that he was not involved in that," Ms Mettam told reporters.
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.
