Photo courtesy of Oxford Properties Group

In the largest office lease of the year in downtown Boston, a tech company that went public last fall is expanding its offices to 256,000 square feet.

Automated marketing software provider Klaviyo renewed its lease at 125 Summer St. with landlord Oxford Properties Group while expanding within the low-rise portion of the 475,000 square-foot complex near South Station.

The transaction marked the single largest office lease of the first quarter in downtown Boston, according to brokerage JLL.

Klaviyo initially leased 160,000 square feet at the property in 2019, relocating and expanding from 225 Franklin St.

The latest transaction helped Boston’s office market register its third consecutive quarter of declining vacancies, with class A office buildings’ vacancy dropping in the first quarter by 0.5 percent to 19.8 percent, JLL reported.

Klaviyo raised $9.2 billion in an IPO last September, when it priced shares at $30. On Monday, following the latest selloff in the stock market, shares declined to $26.43.

The company reported revenues of $585 million in the 12 months ending in June 2023, up 56 percent.

Klaviyo did not respond to a request for comment.