Gallas signed a four-year contract with Arsenal, where he took the number 10 shirt vacated by the recently retired Dennis Bergkamp.
In an October 2006 interview with the official team magazine, Arsenal manager Arsène Wenger explained his decision to hand him the squad number 10 (traditionally a forward's or a midfielder's number): "Number three was uncomfortable for him and I had given the rest of the numbers out.
In the end I thought it might be a good idea to give the number ten to a defender, because a striker would suffer a lot with the comparison with Dennis.
At first I was reluctant to give Dennis's number out, and especially to a defender, but overall I think it's better that way."

Gallas made his full debut for the Gunners at left-back in the team's 1–1 home draw against Middlesbrough on 9 September 2006 and scored his first goal against Sheffield United on 23 September 2006.

Gallas was confirmed as Arsenal captain on 9 August 2007, with Kolo Touré serving as vice-captain.
This caused some controversy, however, as Gilberto Silva was expected to receive the captain's armband.
His first competitive game as Arsenal captain was on 12 August 2007 against Fulham, which ended in a 2–1 victory for Arsenal.
Gallas suffered a groin injury in the game against Blackburn Rovers on 19 August, but returned to play for Arsenal in their 2–0 victory over Bolton Wanderers on 20 October.
In the traditional Arsenal–Manchester United clash at the Emirates Stadium on 3 November, he scored a last-minute goal to give the Gunners a draw.

On 24 November 2007, Gallas scored against Wigan Athletic when Bacary Sagna crossed in and Gallas headed past goalkeeper Mike Pollitt.
On 16 December 2007, Gallas scored against his old club Chelsea, which was to be the only goal in the match to secure three points for Arsenal.
He later reflected on his goal against former club Chelsea, claiming that, "When Cesc [Fàbregas] sent the corner in for my goal, I saw Petr Čech come off his line and to be honest I was sure he would catch it.
So I was surprised when he didn't take it, and it was just a reflex reaction to head it into the goal.
I just happened to be there at the right time."

Gallas attracted some criticism as a result of his actions in a 2–2 draw against Birmingham City on 23 February 2008, a game in which his teammate Eduardo had his leg broken following a challenge by Martin Taylor.
When Gaël Clichy of Arsenal conceded a penalty in injury time at the end of the second half, Gallas walked into the Birmingham half, apparently in protest.
He stood there as Birmingham striker James McFadden converted the penalty.
He then had to be restrained as he appeared to confront the crowd in fury.
After the final whistle, Gallas sat down on the pitch as the rest of the players left the field before being consoled by manager Arsène Wenger.

Gallas scored for Arsenal against Bolton at Reebok Stadium in a 2–3 win on 29 March 2008, starting the comeback which ten-man Arsenal ultimately completed.
Amid speculation that Gallas would be replaced as club captain following his behaviour during the Birmingham game, Wenger said that he would consider the matter at the end of the season.

In August 2008, it was announced that Gallas would remain the club's captain for the 2008–09 season.
In the 2008–09 season, Gallas proved prolific in the Champions League.
He scored six goals in Arsenal's campaign, including two against Dutch side Twente and an equaliser against Dynamo Kyiv, as well as a header against Spurs.
On 29 October 2008, he scored his first league goal of the season against Tottenham at the Emirates, scoring Arsenal's second goal in a 4–4 draw.
On 8 November 2008, in a 2–1 victory over Manchester United, he received praise for his display whilst playing alongside former United defender Mikaël Silvestre.
It was claimed he outshone United defenders Rio Ferdinand and Nemanja Vidić.

In November, Gallas gave an interview to the Associated Press in which he revealed tensions within the squad which were disruptive to team morale, and suggested that Arsenal's younger player
Gallas signed a four-year contract with Arsenal, where he took the number 10 shirt vacated by the recently retired Dennis Bergkamp. In an October 2006 interview with the official team magazine, Arsenal manager Arsène Wenger explained his decision to hand him the squad number 10 (traditionally a forward's or a midfielder's number): "Number three was uncomfortable for him and I had given the rest of the numbers out. In the end I thought it might be a good idea to give the number ten to a defender, because a striker would suffer a lot with the comparison with Dennis. At first I was reluctant to give Dennis's number out, and especially to a defender, but overall I think it's better that way."
Gallas made his full debut for the Gunners at left-back in the team's 1–1 home draw against Middlesbrough on 9 September 2006 and scored his first goal against Sheffield United on 23 September 2006.
Gallas was confirmed as Arsenal captain on 9 August 2007, with Kolo Touré serving as vice-captain. This caused some controversy, however, as Gilberto Silva was expected to receive the captain's armband. His first competitive game as Arsenal captain was on 12 August 2007 against Fulham, which ended in a 2–1 victory for Arsenal. Gallas suffered a groin injury in the game against Blackburn Rovers on 19 August, but returned to play for Arsenal in their 2–0 victory over Bolton Wanderers on 20 October. In the traditional Arsenal–Manchester United clash at the Emirates Stadium on 3 November, he scored a last-minute goal to give the Gunners a draw.
On 24 November 2007, Gallas scored against Wigan Athletic when Bacary Sagna crossed in and Gallas headed past goalkeeper Mike Pollitt. On 16 December 2007, Gallas scored against his old club Chelsea, which was to be the only goal in the match to secure three points for Arsenal. He later reflected on his goal against former club Chelsea, claiming that, "When Cesc [Fàbregas] sent the corner in for my goal, I saw Petr Čech come off his line and to be honest I was sure he would catch it. So I was surprised when he didn't take it, and it was just a reflex reaction to head it into the goal. I just happened to be there at the right time."
Gallas attracted some criticism as a result of his actions in a 2–2 draw against Birmingham City on 23 February 2008, a game in which his teammate Eduardo had his leg broken following a challenge by Martin Taylor. When Gaël Clichy of Arsenal conceded a penalty in injury time at the end of the second half, Gallas walked into the Birmingham half, apparently in protest. He stood there as Birmingham striker James McFadden converted the penalty. He then had to be restrained as he appeared to confront the crowd in fury. After the final whistle, Gallas sat down on the pitch as the rest of the players left the field before being consoled by manager Arsène Wenger.
Gallas scored for Arsenal against Bolton at Reebok Stadium in a 2–3 win on 29 March 2008, starting the comeback which ten-man Arsenal ultimately completed. Amid speculation that Gallas would be replaced as club captain following his behaviour during the Birmingham game, Wenger said that he would consider the matter at the end of the season.
In August 2008, it was announced that Gallas would remain the club's captain for the 2008–09 season. In the 2008–09 season, Gallas proved prolific in the Champions League. He scored six goals in Arsenal's campaign, including two against Dutch side Twente and an equaliser against Dynamo Kyiv, as well as a header against Spurs. On 29 October 2008, he scored his first league goal of the season against Tottenham at the Emirates, scoring Arsenal's second goal in a 4–4 draw. On 8 November 2008, in a 2–1 victory over Manchester United, he received praise for his display whilst playing alongside former United defender Mikaël Silvestre. It was claimed he outshone United defenders Rio Ferdinand and Nemanja Vidić.
In November, Gallas gave an interview to the Associated Press in which he revealed tensions within the squad which were disruptive to team morale, and suggested that Arsenal's younger player
William Gallas