The only hotel in Copacabana or Ipanema that is actually on the beach (all the rest are across the street from the beach). But be warned, this is a no-frills hotel.

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Frommer's Review
The only budget-priced oceanfront hotel in Ipanema, the Arpoador Inn enjoys a privileged location on a quiet stretch of beach popular with the surf crowd, and just around the corner from Copacabana. Even better, the beach in front of the hotel is closed to cars and is therefore pleasantly quiet. The deluxe rooms all face the ocean. The furniture is a little dated but the rooms are bright and spotless. Obtaining these does require booking ahead; if they're full, the superior rooms, which look out over the street behind the beach, make an acceptable alternative. The only rooms to avoid are the standard ones, which are very small, dark, and look into an interior wall.