The 16- and 17-year-old boys were in Warley Woods, Smethwick, where they were confronted by a group of up to nine youths between 9pm and 9.30pm.
They were both punched and kicked as items including mobile phones, ear pods and jewellery were stolen.
It was claimed the youths had a knife although one was not produced.
The victims both suffered facial injuries in the attack with one having to be treated at hospital.
The youths responsible are described as black, aged between 14 and 18 years old, and believed to have congregated near the play area just before the robberies.
West Midlands Police are now appealing for information and believe there will have been people in the park - which is well used by dog walkers - who may have information.
Contact West Midlands Police via Live Chat on their website and quote 20/399700/22.