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.

The two-and-a-half minute clip - which sees Daniel Craig reprise his role as the superspy for a fourth time - gives a closer look at the film after a teaser was released in March.
Featuring car chases, exploding planes and shady crime organisations, it bares all the hallmarks of classic Bond.
Spectre will be released in UK cinemas on 26 October.
The new trailer begins with a first look at Ralph Fiennes in his new role as MI6 boss M, questioning Bond about a rogue mission to Mexico City.
The spy is then seen in Rome, where he meets Lucia Sciarra - played by Monica Bellucci - the widow of an infamous criminal.
After a suggested quick tryst with Sciarra, Bond infiltrates a secret meeting and uncovers the existence of the crime organisation known as Spectre.
Having previously only seen him as a shadowy character in the teaser clip, Christoph Waltz is also finally revealed as Bond villain Franz Oberhauser.
There is also a first glimpse of Sherlock star Andrew Scott, who stars as Max Denbigh, the new head of the Centre for National Security.
Fans on Twitter were quick to give their reaction to the new trailer and many noted it featured some of the theme to 1969's On Her Majesty's Secret Service.
"This looks absolutely stunning!" Ben Peyton wrote, while Rene Friedrich said it was "so much better than the teaser. Finally, how it should be: a blend of iconic, new, fast, quotable".
However, Dekan Apajee said: "As trailers go this is more of a spoiler."

Emergency services were called to Longtown Bridge, near Carlisle, on Tuesday after a 32-year-old man got into difficulties in the water.
The man had just rescued a young girl from the river, Cumbria Police said.
Chief Fire Officer for the county, Jim Onions, warned people against swimming during heatwaves because of strong currents.
Divers and rescue crews were involved in the search, along with the coastguard and an RAF helicopter with thermal imaging equipment.
The body has not been formally identified.
Mr Onions said: "We are really worried that during a hot spell - especially in the school holidays - people enter the water for a cooling dip because the water looks calm on the surface.
"But I've seen highly skilled firefighters in the water struggling because of strong undercurrents."
The rescued girl is at Cumberland Infirmary receiving treatment.
Police have arrested a man they believe held a family and housekeeper hostage in their Washington DC home before killing them.
[NEXT_CONCEPT]
The first full-length trailer for the 24th James Bond film, Spectre, has been released online.
[NEXT_CONCEPT]
A body has been recovered from a Cumbrian river after a search for a man missing in the water.