I received a planter as a gift and the sides are made of untreated wood. Is there anything safe to use to protect them from rotting?

Will this be for indoors or outdoors?

How large is it, and were you planning to fill it up with dirt and plant in it, versus setting another container inside it?

Treated or not, the wood isn't going to hold up for very long if it is exposed directly to dirt and water. Stuff used to treat the wood to resist rot is probably pretty scary stuff, unless you line it with a plastic tarp or bag or something to minimize the impact.