The attacker launched at the two officers with a knife but they avoided injury thanks to the protective vests they were wearing.
Officials in the Belgian capital said there was no immediate indication the attack was terror-related but the location of the incident will raise fears it was Islamist-inspired.
Molenbeek was the focus of operations late last year and in early 2016 to root out extremists based in Brussels.
A spokeswoman for the Brussels prosecutors office said the assailant had said nothing and the case had not been transferred to counter-terrorism specialists.
Several of the attackers involved in the Islamic State-organised Paris massacre last November lived in the poor neighbourhood.
Police spokeswoman Julie Mampuy told AFP: “Two patrolmen were attacked with a knife by a man while they were in the Bonnevie park in Molenbeek.
“They received several knife blows but they were protected by their bullet-proof vests and were fortunately not hurt.”
Brussels has been on high alert since terrorists attacked the city’s airport and metro system in March.