It happened Thursday at 243 East 77th St. just before 1:30 p.m.
Dr. Ronald Goldstein says he was with a patient in the exam room when the attacker walked past the receptionist and punched him.
Goldstein, 62, fell to the floor and passed out while the patient in the chair cowered in fear.
He said he did not know his attacker, but isn't so sure if it was a random act because it seemed deliberate.
"It's a very strange shock to have someone come into our office and invade our space and assault me like that," he said. "No one here recognizes him - we don't have any disgruntled patients, so it was a very strange situation."
Goldstein was rushed to the hospital where he got seven stitches in his forehead.
Seconds after the attack, staff members working in the office saw the suspect walk out as casually as he walked in.
They chased after him to take his picture and because of their quick actions, police have photos of the suspect they are looking for.
Police are still searching for the suspect, who is believed to be in his 20s.
The doctor said he has been at that current location for 15 years and says nothing like that has ever happened. He has had his practice for 37 years and has always practiced on the Upper East Side.
Goldstein was back at work Friday, but taking precautions like locking the front door until a patient has to enter.
"Not in my vaguest imagination would I ever think someone would come in here and hit me, it's just not that kind of profession," Goldstein said.
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