It entered the index with a market value of just under £6 billion, having been floated for £3.3bn.
This surge led to accusations that it had been sold off on the cheap, although this was dismissed by Business Secretary Vince Cable.
The other newcomer is Ashtead, with chemicals group Croda International and miner Vedanta Resources relegated from the shares premier league. The reshuffle was based on the closing prices of companies on December 13.
Royal Mail's life as a top-flight company began in lackluster fashion as its shares opened lower at 589p amid thin pre-Christmas trading.