Nope not an ant, as photographer/entomologist Alex Wild explains (my emphasis):
The delicate build and sabre-like pedipalps of this Brazilian clubionid spider help it to mimic the painfully-stinging trap-jaw ants of the genus Odontomachus. Photo from Morretes, Paraná, Brazil.
What a great example of mimicry! Count the legs (I think one is missing, but there are clearly >6!). I’m not sure which species they mimic, but here’s an Odontomachus brunneus (an ant!) from Wikipedia: