A serial arsonist who broke into and set fire to mobile phone towers across Perth's northern suburbs is being sought by police.

Police believe the man deliberately lit fires at telecommunications sites that have mobile phone towers and satellite dishes in Malaga, Dianella and Morley between July and October.

All three fires were set between midnight and 7:00am, police said.

The man forced entry into the sites, which are fenced-off, before starting fires inside the communication huts, damaging electrical components.

He is described as being 25 to 35 years old with an olive complexion, approximately 175 to 185 centimetres tall, with a medium build and a full beard.

Police said he was seen crossing Reid Highway from Camboon Road in Malaga about 6:15 am on Saturday, October 29.