Sussex Police said the 13-year-old was last seen on Sunday (13 August) at 7pm and it believed she got on a train from Lancing heading west towards Littlehampton or Bognor at about 7.30pm.
Trinity is white, 5’ 3” slim with black shoulder length hair and was wearing a white and black hooded top, black leggings, red trainers and carrying a black bag.
PC Lucie Collier said: “We are concerned about Trinity due to her age. We believe she could be in the Bognor area particularly the Dorset Road and Collyer Avenue area so if anyone sees her or has any information about her please get in touch.”
Anyone with information is asked to contact police online or call 101 quoting serial 1373 of 13/08.