The victim was allegedly threatened with a large machete as he was approached by a large group while cycling through Willenhall Memorial Park, in Willenhall, at around 6.20pm on Saturday.
The suspect remains in custody after he was arrested from an address in Walsall on Saturday night.
A spokesman for West Midlands Police said: "We've arrested a 16-year-old boy over allegations he threatened another boy with a machete in a park before stealing his bike.
"The victim was cycling through Willenhall Memorial Park at around 6.20pm Saturday when he was approached by a group.
"One of the group produced a large machete and threatened to stab the victim unless he handed over the mountain bike.
"Enquiries led us to an address in Walsall last night where the teenager was arrested on suspicion of robbery; he remains in police custody."