Daron Dylon Wint, 34, who had previously worked for the victims' family business, was detained late Thursday in the US capital.
He is accused of killing the four victims and setting the house on fire.
Police refused to confirm reports Mr Wint was identified by DNA from the crust of a pizza delivered to the home.
Savvas Savopoulos, 46, chief executive of American Iron Works, was found dead in his family's home in Woodley Park, northwest Washington, just blocks away from the home of Vice-President Joe Biden.
Police discovered his body alongside those of his wife Amy, 47, his 10-year-old son Philip and housekeeper Veralicia Figueroa, an El Salvador national.
A week after the gruesome discovery, investigators have provided few details in a case that has shocked the city and its high-profile social circles.
There have been media reports of a $40,000 cash drop being made to the home hours before the incident, and a text message from Mrs Savopoulos telling a member of staff not to come into work that same day.
The Washington Post reported that a delivery order was placed with a local pizza company while the family was being held hostage and DNA found on a crust has been matched to Mr Wint.
Police officials would not confirm or deny this report to the BBC.
Police have arrested a man they believe held a family and housekeeper hostage in their Washington DC home before killing them.