He made his first Premier League appearance on the opening day of the 2012–13 season, setting up a goal and wearing the captain's armband in a 3–2 defeat against 2011–12 champions Manchester City. He scored his first Premier League goal in a 4–1 defeat away to West Ham United. His second goal of the season came on 25 November, as he opened the scoring in a 2–0 home win against Newcastle United. The 50th goal of his career came in a 2–0 victory at Reading. On 11 April 2013, Lallana signed a new five-year contract with Southampton.
In Southampton's 4–1 Premier League victory over Hull City on 9 November 2013, Lallana started with the ball 30 yards from goal and dribbled his way past five players into the left-hand side of the 18-yard box, before slotting it past Hull goalkeeper Steve Harper. On 18 April 2014, he was named as one of the six players on the shortlist for the PFA Players' Player of the Year award. On 27 April 2014, he was named alongside teammate Luke Shaw in the PFA Team of the Year. On 6 May, he was crowned as both the Southampton Players' Player and Fans' Player of the year.
On 7 November 2013, along with Southampton teammates Rickie Lambert and fellow uncapped Saint Jay Rodriguez, he was called up to the England team. Lallana won his first cap for England on 15 November by starting a friendly match against Chile at Wembley Stadium. Although England lost 2–0, Lallana's performance received generally favourable reviews. He also started in England's defeat against Germany four days later. He gained his third cap in a 1–0 friendly win against Denmark on 5 March 2014, coming on as a substitute for Jack Wilshere and crossing for Daniel Sturridge to head in the only goal of the match.
Lallana married his fiancée Emily Jubb on 24 December 2013. The couple, who had been dating for seven years, were due to marry on 14 June 2014, which coincided with England's opening World Cup match with Italy. They wed at Poole Register Office in Dorset. Together, they have two sons, Arthur Michael, born on 26 September 2012 and Albie George born on 3 June 2015.