The marriage is her third. Lewthwaite is the widow of London Underground bomber Germaine Lindsay and is believed to have been married to al-Shabaab Abdi Wahid, who officials presume to be dead.
Lewthwaite has been tied to a series of terrorist attacks. She is believed to have been involved in a failed plot to blow up hotels and a mall in Mombasa, Kenya. She has also been linked to the terrorist attack in a Nairobi mall that left 67 people dead.
After Nairobi, it is believed she slipped back into Somalia. In October, a picture surfaced of her holding her newborn daughter, Surajah, her fourth child and second with Wahid.
The marriage will probably mean that Lewthwaite will be offered protection from Hassan's relatives.
"Her in-laws will treat her very well as she is now one of them and part of one of the large clans," a security official told the Mirror.