Per Mertesacker is en route to London to undergo a medical at Arsenal, Press Association Sport understands.
The Gunners have been in talks with Werder Bremen over the signing of Germany defender Mertesacker, who was given permission by club and country to jet out to England.
Arsenal are also understood to be close to concluding a deal for Brazil left-back Andre Santos as they sought to revamp their defence following Sunday's humiliating 8-2 defeat at Manchester United.
The signings would be music to the ears of Gunners fans, who have long urged manager Arsene Wenger to strengthen a rearguard that has, at times, been left woefully exposed in the past year.
Mertesacker, 26, would also provide much-needed height and experience at the back, standing at 6ft 6in and boasting 75 international caps.
Reports suggest he could be available for as little as around £8million because of Bremen's financial problems. Wenger has long been linked with Mertesacker, who is currently on international duty.
Germany boss Joachim Low said: "Per contacted me last night and informed me (about Arsenal's interest). I told the player that I would release him for the day."
Arsenal have also agreed a fee with Turkish giants Fenerbahce - reportedly £6.2million - for Santos, with the move subject to a medical and personal terms being agreed. The 28-year-old would help fill the void left by the sales of Gael Clichy to Manchester City and Armand Traore to QPR.
South Korea captain Park Chu-Young is expected to complete his move from Monaco later on Tuesday, while the Gunners have been linked with a host of other names ahead of Wednesday's transfer deadline, including Chelsea's Florent Malouda, Alex and Yossi Benayoun.
They also last week saw a bid rejected for Bolton's Gary Cahill, a pursuit they may abandon should a deal for Mertesacker be concluded.