John A. Catsimatidis, the billionaire owner of the Gristedes grocery chain, stood on the steps of City Hall on Tuesday and announced that he would seek the Republican nomination for mayor of New York City.

Then he started talking about his suit.

“I think my wife paid $100 for this jacket,” Mr. Catsimatidis, who bills himself a “common billionaire,” said as he gripped the lapel of a plus-size Jos. A. Bank blazer. “I’m not wearing a $5,000 suit.” He warned his daughter, standing nearby, that he would not be buying her an $80 million apartment. Later, he added, “I feel the people’s pain.”

“There were times in the ’80s,” Mr. Catsimatidis, a 64-year-old Upper East Sider, said, “when I was afraid to walk from Fifth Avenue to Madison Avenue.”

It was an unconventional announcement from an unconventional candidate, a corporate dealmaker with no political experience, no natural constituency and little support among the city’s chattering class.

But Mr. Catsimatidis, who appeared to be enjoying himself immensely during Tuesday’s event, does have an estimated $3 billion fortune — more than enough to mount a formidable advertising operation and, at the very least, be a thorn in the side of a crowded Republican field of candidates who are little known to the public.

An immigrant from Greece who made much of his fortune in the oil business, Mr. Catsimatidis (pronounced Cah-tsee-mah-TEE-dees) said he wanted to give back to a city that had served him well. “I’m a visionary; I’m not a maintenance person,” he declared, going on to say he understood the challenges facing small businesses, partly because his grocery trucks often incur thousands of dollars’ worth of ticket fees.

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