MIAMI—The challenges swirling around a skyscraper known as One Brickell City Centre, which at around 1,000-feet high would be Miami’s tallest corporate tower, show how the city’s once-sizzling office market is starting to cool.  

New York developer Related Cos. and Swire Properties, an international development firm founded by the British Swire family, are struggling to find an anchor tenant roughly a year after the groundbreaking. Related is restructuring its agreement with Swire, which owns the land, according to people familiar with the matter.

Swire even has considered selling the 1.55-acre site in Miami’s downtown, according to a document viewed by The Wall Street Journal.

Swire said it “continuously evaluates different options for its development sites” and that the one for One Brickell City Centre “is currently not for sale.” Related said it was continuing to work with Swire.

Rising interest rates and hybrid work have punished the U.S. office market. Miami for years weathered such headwinds better than most cities, thanks in part to a steady stream of corporate relocations and limited supply of office buildings.

Now, Miami’s exceptionalism appears to be fading. Leasing activity in the Miami office market was down 25% last year from 2022, according to commercial real-estate services firm JLL. Sublease vacancy increased by 66% through the end of the year, a sign that existing tenants want to cut back on space.

“There’s a slowdown of new-to-market activity,” said Steven Hurwitz of JLL. “We’re sort of at a new inflection point.”

Miami office-construction starts have slowed down after a recent peak in the second quarter of 2023, according to Juan Arias, director of market analytics for CoStar in South Florida. He said that new construction was being affected by higher interest rates, elevated costs for construction material and labor, and a slowdown in leasing activity amid a softer economic environment.

“The office boom is slowing down, and new company relocations are also slowing,” Arias said.

Miami is trying to pivot from an economy long geared toward leisure and tourism into a new business and financial capital. The city can claim many successes, attracting financial and tech firms from the Northeast and other parts of the country.

But Miami is also suffering its most acute case of growing pains since the pandemic accelerated the business boom. Single-family home and rental prices have risen in Miami more than most anywhere else, and property insurance costs are soaring. Those trends make it harder for employees to move to the city. Executives who do relocate to Miami find that the top private schools are at capacity.

Even with the recent choppiness, Miami’s office market remains one of the country’s strongest. It finished last year with the highest annual office rent growth of all major U.S. markets. The city’s annual net absorption, which looks at how many tenants moved in versus moved out, was the nation’s third highest, netting more than 500,000 square feet of occupied space. The office vacancy rate of 8.4% is among the lowest in the U.S.

Related chairman Stephen Ross is among those whose interest in new South Florida projects has intensified. One Brickell City Centre, the 1.5 million-square-foot tower with 68 floors, is one of his more ambitious efforts. At the groundbreaking, Ross called it “the most important project in Miami,” while Swire president Henry Bott said the building would be “a beacon” for the city, with outdoor terraces on almost every floor.

Ross had pledged to bring in big-name tenants to the tower, which is scheduled for completion in 2028. He spoke with Ken Griffin about moving his hedge-fund firm Citadel to the site, according to people familiar with the matter.

In November, the two moguls seemed to deepen their relationship over a mutual interest in football. Griffin was in negotiations to buy a minority stake in the Miami Dolphins, in which Ross is the majority owner, according to people familiar with the matter.

Now the Citadel chief executive is no longer in discussions for a Dolphins stake. His company also passed on becoming the anchor tenant at One Brickell City Centre in favor of developing the waterfront site Griffin bought in 2022, where his plans include a luxury hotel for the top of the office tower.  

“I’m not doing anything with Ken Griffin,” Ross told the Journal last month.

Still, the two billionaires indicated they could work together in the future. In a joint statement, Griffin and Ross said they “look forward to partnering together on initiatives that will benefit the community.”