The victims were found in raids at three restaurants in Coventry, Wolverhampton and West Bromwich and an address in Handsworth, Birmingham.
Police suspect the men were working for "next to nothing".
A 49-year-old man and a 45-year-old woman have been arrested on suspicion of slavery offences.
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The victims are thought to be mainly Slovakian and Romanian.
The suspects, who are both Slovakian, have been arrested on suspicion of providing staff through a recruitment consultancy.
The raided restaurants were Ming Moon, Stafford Road in Wolverhampton, Wing Wah, Fletchamstead Highway in Coventry and Red Leaf, High Street in West Bromwich.
Five suspected slavery victims were found at the Chinese restaurant in Wolverhampton, 12 at Wing Wah Coventry and three at the an address on Anthrobus Road in Handsworth.
The men will be given the opportunity to speak to specially trained officers and helped into alternative accommodation.
West Midlands Police said signs of modern slavery to look out for include "large groups of people staying in multi-occupancy houses and being transported to and from addresses in vans or minibuses from early in the morning and not coming back until late at night".
Further investigations are underway, the force added.
Twenty men have been found living in "poor conditions" during four simultaneous raids across the West Midlands, police said.