No, but some things come reasonably close and could, in the right circumstances, float in air.

There’s no solid material with a density less than standard atmospheric density air on Earth. The primary reason for this is that air is, even for a gas, pretty low density. The major components, oxygen and nitrogen are waaay above their boiling points and have quite long mean-free paths meaning that the particle density is very low.

So in an ideal gas, there is 1 mole of gas per 24.4 litres at room temperature and standard pressure. This means in a litre of air there are just shy of 40 millimoles of air present or ~2.47 × 10^22 molecules. If you were to distribute these evenly through a cube the average separation between the molecules would be around 34 angstroms (3,433 picometers).

Now, take a very low density uniform solid, lithium being the least dense of the solid elements at stp. Lithium atoms in a solid block of lithium are less than 3 angstroms apart, so in a particular box, you’re going to pack far more atoms of lithium in a given volume than you are of nitrogen and oxygen, making the block of lithium far denser than air.

If you cheat slightly you can get solid materials that have densities very close to that of air if you make them primarily out of air. So one example would by polystyrene, which can be 98% air by volume:

Aerogels can go much higher than this and approach 99.9% air.

Graphene aerogel is the current record holder at 0.16 milligrams per cubic centimetre. In reports it is cited as being “7 times lighter than air” but this is misleading as if it were lighter than air it wouldn’t be possible to weigh it outside a vacuum chamber. Once you remove the buoyancy effect of air (yes, air causes buoyancy on low density samples), the sample has essentially only fractionally higher density than air.

Aerogels can also support heavy loads:

If you were able to place a very thin gas-impermeable film around an aerogel that was instead filled with helium or if you were to increase the air pressure in a space or have the air inside the aerogel be hotter than the ambient air (before you sealed it), you could quite easily get an aerogel to float.

But you know what else you could use to do that same trick?

A balloon.