A 15-year-old girl missing from Sydney's north shore has been found.

The girl's family initially reported her missing on Friday after she did not show up at school.

The girl was last seen at Artarmon railway station at 8.20pm on Thursday, and her family and police held serious concerns for her welfare as her disappearance was out of character.

It's unclear where the girl was meant to be on Thursday night, and a police spokesman said officers were looking at the possibility she was with someone.

About 12.45pm on Monday, the girl was found safe and well in Artarmon. The police thanked the community for their assistance.