[6] His achievements and footballing potential attracted Arsenal, ultimately moving to the Premier League side for a fee of £8.45 million the following season.

[7] He had established himself as a core component in the Arsenal first team, as well as contributing with a host of individual performances, including aiding in ending the club's nine-year trophy drought, scoring a crucial equalizer en route to winning the FA Cup in 2014.

[6] On 16 June 2009, newly promoted Ligue 1 side FC Lorient signed Koscielny on a four-year contract for around €1.7 million.

Koscielny became a fan favourite when he scored his second goal with a header deep in injury time against Montpellier in a 2–2 draw after a cross from Olivier Monterrubio.

An impressive season culminated in reports from French daily Ouest-France that the defender was training with the Arsenal reserves ahead of an official announcement.

Arsenal's initial offer for Koscielny was believed to be in the region of £4 million, while they were then prepared to do a deal that would also involve youngster Francis Coquelin moving on loan in the opposite direction, which Lorient rejected.

Arsenal confirmed the signing on 7 July 2010 and Koscielny was handed the club's number 6 shirt, previously worn by Philippe Senderos, who had moved to Fulham earlier in the summer.

Wenger explained he bought Koscielny due to the fact he added height to the team and Arsenal were low on centre backs.

[15] Koscielny made his debut in European competition on 15 September 2010, when he partnered Sébastien Squillaci in a 6–0 win over Braga at the Emirates Stadium.

Koscielny was praised for performances against Sunderland and made a brilliant tackle against Tottenham Hotspur to stop Aaron Lennon from scoring during their Football League Cup tie.

On 27 October, he blocked a shot off the line and provided an assist to Theo Walcott against Newcastle United in the fourth round of the League Cup, where Arsenal went on to win the match 4–0 at St James' Park.

[16] On 27 February, in the last minute of the 2011 Football League Cup Final, Koscielny was involved in a mix-up with his goalkeeper Wojciech Szczęsny, which resulted in Birmingham City striker Obafemi Martins scoring the winning goal.

[22] On 30 October, he scored a header from a corner in the 88th minute against Reading in the Football League Cup, helping Arsenal come back from 4–0 down to eventually win the game 7–5.

[26] Koscielny started the 2013–14 season by conceding a penalty and getting sent off in Arsenal's 3–1 Premier League opening day defeat at home to Aston Villa.

[27] However, his partnership with Per Mertesacker saw the Gunners achieve the joint-best defensive record in the Premier League at the halfway point of the season, as the team topped the table.

[30] On 17 May, Koscielny scored Arsenal's equalising goal as the team came from 2–0 down to defeat Hull City 3–2 in the 2014 FA Cup Final, winning his first piece of silverware with the club.

[32] After sustaining an achilles tendon injury during the October international break, Koscielny was absent from the Arsenal team for seven weeks.

[33] On 29 November, he returned to Arsenal's starting line-up alongside Per Mertesacker in central defence, as the Gunners kept a clean sheet in a 1–0 victory against West Bromwich Albion at The Hawthorns.

[38] On 30 May 2015, Koscielny was selected to start in the 2015 FA Cup Final, playing the full 90 minutes in a 4–0 defeat of Aston Villa at Wembley Stadium.

[39] Koscielny started every game in the first month of the season, excluding a 0–0 draw with rivals Liverpool at Emirates Stadium,[40] due to an injury.

This ended after a 3–2 defeat to Manchester United, when Koscielny missed four games through injury, including a North London derby at White Hart Lane.

[47][48][49] On 12 January 2017, Koscielny, alongside teammates Francis Coquelin and Olivier Giroud signed a new long-term contract with Arsenal.

[54][55] On 3 May 2018, Koscielny suffered an Achilles injury during the UEFA Europa League semi-final second leg match against Atlético Madrid, and was ruled out for at least six months.

[59] On 11 July 2019 it was reported that, with a year left on his contract, he refused to take part in the Arsenal pre-season tour of the United States and decided to stay in the UK in order to force a move to Bordeaux.

[60] On 6 August 2019, Koscielny signed for French Ligue 1 club Bordeaux, in a deal rumored to be worth £4,600,000, after nine years with Arsenal.

[62] On 31 January 2022, Bordeaux officially moved him into a non-playing role for Koscielny to "contribute to the development of the club's international partnerships".

[68] He made his FIFA World Cup debut as a substitute for the injured Mamadou Sakho in a 5–2 defeat of Switzerland in the team's second group match.