Yes, a widowed princess retains her title by way of becoming the Dowager Princess. Catherine of Aragon became the Dowager Princess of Wales when her first husband, Prince Arthur died. After Henry VIII married her, she became Queen of England. When Henry divorced her later in life, he insisted she be called the Dowager Princess and afforded every courtesy of that title.
Catherine is said to have incensed Henry at every turn by continuing to call herself Queen and refusing to answer to Dowager Princess.