It is both. We normally think of it as a base because it raises the pH of water by accepting a proton,
HCO3- + H2O <-----> H2CO3 + OH-
but if you add bicarbonate to a strongly basic solution, it actually lowers the pH by donating a proton.
HCO3- + OH- <------> CO3(2-) + H2O. That's why it is an amphoteric substance
