Mew cannot be caught in "Pokémon Ruby." There is one game in Generation III that Mew can be captured in, however, and that's "Pokémon Emerald." You'll need a copy of that game to catch him, and you can then transfer him to "Ruby."Continue Reading
Getting access to Mew in "Pokémon Emerald" requires yet another event item from years past: the Old Sea Map. You might find someone who is willing to trade you the item, but it's more likely you need to hack it into the game with the aid of a cheat device such as the Action Replay.
With the Old Sea Map in hand, you can now talk to the woman in front of the S.S. Tidal to be taken to Faraway Island. This is the only means of getting to the island without cheating. Mew is found in the forest on the island and must be chased for a bit before it engages in battle with you. Mew can then be weakened and captured.
Another Gameboy Advance is needed to make the swap. You also need to have the National Pokédex in "Pokémon Ruby" to be allowed to trade Mew.