Catherine Shaw, from Witney, Oxfordshire, who has been travelling since September last year and has previously visited Mexico and California in the US, was reported missing on March 5.
She was last seen on March 4 and had been staying in the San Pedro, Lake Atitlan area of the central American country with a friend.
Her belongings were left behind, the Lucie Blackman Trust said.
In a statement released through the trust, which is supporting the family, her parents said they have been “desperately worried” since her disappearance.
“She has always been really good about keeping in touch and informing us of her whereabouts and activities,” they added.
“So this is unusual behaviour which gives us great concern for her safety. Please help us to find her.”
A Foreign Office spokeswoman confirmed they are supporting the family of a British woman and are “in contact with the local authorities as they search for her”.
In a video released by the trust, her mother urged her to get in touch as they need to know she is happy and well, while her father described his daughter as of “slim build, and about 5ft 7in tall”.
He added: “Your friends, your family are all really worried about you, please come home sweetheart.”
The Lucie Blackman Trust chief executive Matthew Searle stressed how the “first few days are vitally important” because those who may have seen something could still be in the area.
“We urge anyone who may have any information, no matter how small, to get in touch as soon as they can. They could hold the key to bringing Catherine home,” he added.
The trust said members of her family are travelling to San Pedro to join the search this weekend.
Anyone with any information should call the Lucie Blackman Trust on +44 (0) 1983 718802, then option 1, email [email protected] or, if they are in Guatemala, they can contact local police.