Mami Konneh Lahun, 24, did not return to her accommodation in Greenwich after Sunday’s race.
Ms Konneh Lahun arrived in London on April 7 and had been due to fly home later today.
Police say she has no known links to the UK and does not own a mobile phone.
She is believed to be Sierra Leone’s fastest ever 10,000m and 5,000m runner and last year won the Liberia Marathon.
Anyone with information is asked to call 101, quoting reference 14MIS012462.
Image credit: Metropolitan Police