After his 2014 season had been interrupted by injury, Armitage planned with St Kilda coach Alan Richardon and committed to shedding excess muscle to re-design himself as a player, modeling himself after Hawthorn premiership player Jordan Lewis. In 2015 pre-season training he emphasized his running rather than his weight-lifting and shed five kilograms of weight, which led to a major improvement in his game, and he enjoyed career-best form in the 2015 season. He played almost every game of the season for St Kilda, in August he was one of the top five in the AFL for possessions, and he filled in as captain for three games in the absence of Nick Riewoldt. Already contracted until for 2016, he signed a three-year contract extension to stay at the club until the end of 2019. He finished off the season having played 21 out of 22 games for St Kilda and came second in the Trevor Barker Award, St Kilda's best and fairest, as well as polling 12 votes in the Brownlow Medal.