Police are desperately searching for two teens who appear to have fallen in love and run away together.

Lucas Bollard and Sophie McGregor, both aged 14, haven't been seen since Saturday, when Lucas' mother found a note he left behind.

The two have been dating for three months, and it's believed they may have run away to Sydney’s Blue Mountains, from their homes in Botany Bay.

Police are asking anyone in the Blue Mountains or Lithgow areas to be on the lookout for the teens.

Lucas is 183cm tall and was last seen wearing blue jeans, white sneakers and a grey Hurley hooded jumper.

His girlfriend Sophie is 152cm tall, was last seen wearing black jeans, black boots and a white t-shirt with a cartoon on the front.

Police are concerned the two did not take enough supplies with them, but say they had enough money to get by.

"It's just out of character for both of them and we're just making all the inquiries we possibly can," a police spokesman said.

"They're young kids trying to get away and our concern is that we don't know where that 'away' is."

Police are believed to be monitoring the couple's mobile phone use and social media updates for any clues to their whereabouts.