Gylfi took part in the 2008 U19 European Championship qualifying, scoring twice in qualification and twice in the elite round, but Iceland lost out to group winners Bulgaria by three points and missed out on the finals.
In November 2007, Gylfi made his under-21 debut for Iceland, playing 30 minutes against Germany in a 3–0 loss.
He made his first appearance in the 2009 European Under-21 Championship qualifying four days later, coming as an extra-time substitute, and played in the remaining matches.
In Iceland's last match, he scored the opener against Slovakia, but Slovakia's Miroslav Stoch equalised to make it 1–1.
Iceland failed to qualify for the play-offs but made a bright start to the 2011 campaign.
He made his campaign debut in October against San Marino, scoring a brace within 16 minutes in a 6–0 thrashing.

In May 2010, Gylfi made his full Iceland debut in a convincing 4–0 win over Andorra and provided the assist for the second goal from a free-kick.
Gylfi also helped Iceland's U21 side reach the 2011 UEFA European Under-21 Championship, playing in both play-off legs against Scotland as Iceland won 4–2 on aggregate.
In the second leg, Gylfi scored two second-half goals to ensure Iceland won the match 2–1.

On 13 October 2014, Gylfi scored both goals (one of which was a penalty) as Iceland defeated the Netherlands in a UEFA Euro 2016 qualifying match.
In the reverse fixture the following 3 September at the Amsterdam Arena, he converted another spot-kick for the only goal of the match, after Gregory van der Wiel fouled Birkir Bjarnason.

Gylfi was selected for UEFA Euro 2016.
On 18 June, he scored Iceland's goal in their second group stage match against Hungary, a 1–1 draw at the Stade Vélodrome.

In May 2018, Gylfi was named in Iceland's 23-man squad for the 2018 FIFA World Cup in Russia.
During the group stage match against Croatia, he scored a penalty in the 76th minute, although lost the match 2–1, and was knocked out of the tournament.
Gylfi took part in the 2008 U19 European Championship qualifying, scoring twice in qualification and twice in the elite round, but Iceland lost out to group winners Bulgaria by three points and missed out on the finals. In November 2007, Gylfi made his under-21 debut for Iceland, playing 30 minutes against Germany in a 3–0 loss. He made his first appearance in the 2009 European Under-21 Championship qualifying four days later, coming as an extra-time substitute, and played in the remaining matches. In Iceland's last match, he scored the opener against Slovakia, but Slovakia's Miroslav Stoch equalised to make it 1–1. Iceland failed to qualify for the play-offs but made a bright start to the 2011 campaign. He made his campaign debut in October against San Marino, scoring a brace within 16 minutes in a 6–0 thrashing.
In May 2010, Gylfi made his full Iceland debut in a convincing 4–0 win over Andorra and provided the assist for the second goal from a free-kick. Gylfi also helped Iceland's U21 side reach the 2011 UEFA European Under-21 Championship, playing in both play-off legs against Scotland as Iceland won 4–2 on aggregate. In the second leg, Gylfi scored two second-half goals to ensure Iceland won the match 2–1.
On 13 October 2014, Gylfi scored both goals (one of which was a penalty) as Iceland defeated the Netherlands in a UEFA Euro 2016 qualifying match. In the reverse fixture the following 3 September at the Amsterdam Arena, he converted another spot-kick for the only goal of the match, after Gregory van der Wiel fouled Birkir Bjarnason.
Gylfi was selected for UEFA Euro 2016. On 18 June, he scored Iceland's goal in their second group stage match against Hungary, a 1–1 draw at the Stade Vélodrome.
In May 2018, Gylfi was named in Iceland's 23-man squad for the 2018 FIFA World Cup in Russia. During the group stage match against Croatia, he scored a penalty in the 76th minute, although lost the match 2–1, and was knocked out of the tournament.
Gylfi Sigurðsson