A 21-year-old Perth man faces nine online sex charges after allegedly encouraging a child to commit "indecent acts" over the Internet.

Police say that the man, who is from Armadale, southeast of Perth, sent lewd images to a 13-year-old and encouraged the child to do similar acts.

In addition, the police also accuse the man of trying to convince the child to meet for sex on a number of occasions. Law enforcement officers seized computers, modems and storage devices during a search of the man's Armadale home.

The man was charged with five counts of exposing a person believed to be less than 13 years old to indecent matter. He will also face four counts of attempting to procure a person believed to be less than 13 years old to engage in sexual activity.