Robert Dawson, a former detective sergeant, had been investigating a rape allegation in 2010 when he began "an inappropriate relationship" with a "vulnerable victim", the Met said.
A misconduct panel found his behaviour breached standards of "respect, courtesy duties and responsibilities".
He was dismissed without notice following a hearing on Wednesday.
The panel concluded the former detective's actions breached the Met's code of professional standards "in such a serious way" that it amounted to gross misconduct.
Mr Dawson, who was attached to the specialist crime and operations team, had been found not guilty of wilfully neglecting to perform duty and wilfully behaving with misconduct at Southwark Crown Court on 12 June last year.
A Met Police officer has been sacked for having sex with a vulnerable rape victim whose case he was investigating.