The central bank last bought ETFs on Jan. 28 and held off buying more as Japan's share prices hit their highest in three decades in line with a global stock market rally.
The fact it stepped in on Friday, when Japan's Nikkei suffered its biggest one-day fall since April, underscored the bank's new approach of intervening only when markets become volatile, analysts said.
The BOJ bought 5 million yen ($47,068) worth of ETFs on Friday, central bank data showed.
The BOJ has been buying ETFs for a decade as part of efforts to drag the world's third-biggest economy out of deflation.
Its policy at the moment is to buy ETFs at an average annual pace of roughly 6 trillion yen, a commitment that forces it to keep buying even when stocks are booming.
Some analysts saw the BOJ's moves in February as a prelude to what may come out of a March review of its policy tools to make its asset-buying programme more nimble.
"When stock prices were booming like they did in February, it didn't make sense for the BOJ to keep buying ETFs," said Izuru Kato, chief economist at Totan Research.
"The March review will be mostly about spelling out in language what the BOJ actually did in February," he said.
The BOJ has said its ETF-buying programme will be among tools it could tweak in its policy review.
Sources have told Reuters it will seek ways to slow purchases when markets are stable, while reassuring investors it will step in forcefully when prices are volatile.
($1 = 106.2300 yen)
(Reporting by Leika Kihara; Editing by David Clarke)
By Leika Kihara