Detectives investigating the disappearance of Sarah Everard arrested the officer on Tuesday night in Kent, southern England, according to a police statement.
Everard was last seen on March 3 in Clapham, South London.
Police remain in custody at a London police station, police said.
“A woman was also arrested at the same place, presumably to assist an offender,” the statement said. “She was also arrested at a London police station.”
“The arrest is a serious and important development,” police said.
“We will continue to work at full speed on this investigation, but the fact that the arrested man is a serving Metropolitan police officer is shocking and disturbing,” said Assistant Commissioner Nick Ephgrave.
Detective Chief Inspector Katherine Goodwin said: “This is an important development in our investigation. It is a fast moving investigation and we are doing everything in our power to find Sarah.”