It is understood the man threatened staff with a handgun and fled on foot from the Queensgate branch of Paddy Power with more than £10,000.
Police say the attacker who struck at 9.30am changed clothes twice as he made his getaway.
Officers cordoned off a number of lanes near the shop and officers are studying CCTV footage from cameras in the area.
The police commander for Inverness, Chief Inspector Mark MacKay, said: "Thankfully nobody was injured in the course of this incident.
"Crimes of this nature are relatively rare in Inverness and we will be carrying out high-visibility patrols in the city centre and surrounding areas whilst our efforts continue to identify the person responsible.
"We would urge anyone who may have seen the suspect or has information regarding this crime to contact us as a matter of urgency."
The man is described as 6ft, and of average build.
At the time of the incident he was wearing a black bomber jacket, light blue hooded top, a black balaclava, black cargo trousers and black shoes.
He walked a short distance before changing into a red and black checked hooded top with a white woollen fleece lining, a black fleece lined hat with earflaps, and black trainers with white soles.
After walking another short distance, he changed again into a dark coloured T-shirt, faded black jeans, and removed the hat. He was also carrying a large black holdall.