Arteta among flurry of late Arsenal deals
Leaving Arsenal, however, was Denmark striker Nicklas Bendtner who joined Premier League rivals Sunderland on a season-long loan.
Spanish midfielder Arteta, 29, moved to the London club from Everton for a fee of 10 million pounds.
"I am very disappointed to lose him but the prospect of Champions League football was something I wasn't able to offer him," Everton manager David Moyes told his club's website.
The deal ended a six-and-a-half-year spell at Goodison for the skilful Arteta.
Arsenal manager Arsene Wenger also acquired Germany defender Mertesacker from Werder Bremen and Brazil left back Santos from Fenerbahce on permanent deals while Israel midfielder Benayoun joined from Chelsea on a season-long loan.
The 26-year-old Mertesacker has been capped 75 times by Germany.
Wenger, under pressure to strengthen his squad after picking up just one point from their opening three league matches, also signed South Korea striker and captain Park Chu-young from AS Monaco on Tuesday.
Santos, 28, made 52 league appearances for Fenerbahce and earned the first of his 22 Brazil caps at the 2009 Confederations Cup in South Africa.
Benayoun, 31, joined Chelsea from Liverpool a year ago but his short spell at Stamford Bridge was blighted by injury.
Bendtner, 23, has struggled to win a regular place in Arsenal's first team in recent months.
Wenger does not usually take part in frantic deadline day dealing but after the embarrassing defeat by United on Sunday and the sales of midfielders Cesc Fabregas and Samir Nasri the Frenchman has had to add proven quality to his squad.