Is it required to electrically ground rebar in an exterior concrete slab?
You need to bond the rebar (note the difference!)
The issue at hand here isn't that the rebar is way out at some voltage far above the &quot;earth potential&quot;, whatever that means, but that that the slab and rebar can have a potential on it that's different from the potential of some other nearby conductive object (such as some part of the spa/hot tub).  This potential difference means that someone touching the conductive object while standing on the slab will complete the circuit and equalize the potential between the two objects, getting &quot;bit&quot; in the process.
To keep your hot tub setup from growing shocky teeth, all the non-current-carrying metal parts, as well as that in-slab rebar, need to be bonded together and to the hot tub's central bonding point (located at the hot tub pump most likely) with  #8 copper.  The NEC will let you omit this bonding for self-contained hot tubs that are listed, labeled, and identified for outdoor use without bonding and have rims greater than 28&quot; above all perimeter surfaces within 30&quot; of the perimeter of the hot tub, but I would call it a wise investment to at least have a bonding provision in the slab, in case a future hot tub that gets put in requires the slab to be bonded.