Original Hummel trainingsshirt Middlesbrough FC 2018/19

The 2018–19 transfer window is club manager Tony Pulis' first official summer transfer window as Middlesbrough manager. Pulis' first signing of the window was a defensive Paddy McNair, arriving from rivals Sunderland for £5 million. Bristol City star defender Aden Flint subsequently joined the club for a fee of £7 million. Sam Stubbs and Djed Spence then joined the reserves, both on free transfers, arriving from Wigan Athletic and Fulham respectively. As August emerged, Andy Lonergan joined the club on a free transfer from rivals Leeds United. On 8 August 2018, Middlesbrough completed the signing of Jordan Hugill from West Ham United on a season-long loan deal.

Tomás Mejías, Martin Cranie, Tom Brewitt, Liam Cooke, Liam Hegarty, Jakub Sinior, Keiran Storey, Robbie Tinkler, Jay Wilson and Matthew Elsdon were all released at the end of the 2017–18 season. Antonio Barragán then left the club to join Real Betis permanently, having spent the previous season on loan there, while Mikael Soisalo and Fábio left to join Zulte Waregem and Nantes respectively. Players who were loaned out included Joe Fryer to Carlisle United, Hayden Coulson to St Mirren, Luke Armstrong to Gateshead, George Miller to Bradford City. The club then said their goodbyes to their team superstars: striker Patrick Bamford to Leeds, centre-back Ben Gibson and winger Adama Traoré.