A bstract A measurement of the CP asymmetries S f and $$ {S}_{\overline{f}} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>S</mml:mi><mml:mover><mml:mi>f</mml:mi><mml:mo>¯</mml:mo></mml:mover></mml:msub></mml:math> in B 0 → D ∓ π ± decays is reported. The decays are reconstructed in a dataset collected with the LHCb experiment in proton-proton collisions at centre-of-mass energies of 7 and 8 TeV and corresponding to an integrated luminosity of 3.0 fb −1 . The CP asymmetries are measured to be S f = 0.058 ± 0.020(stat) ± 0.011(syst) and $$ {S}_{\overline{f}}=0.038\pm 0.020\left(\mathrm{stat}\right)\pm 0.007\left(\mathrm{syst}\right) $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>S</mml:mi><mml:mover><mml:mi>f</mml:mi><mml:mo>¯</mml:mo></mml:mover></mml:msub><mml:mo>=</mml:mo><mml:mn>0.038</mml:mn><mml:mo>±</mml:mo><mml:mn>0.020</mml:mn><mml:mfenced><mml:mi>stat</mml:mi></mml:mfenced><mml:mo>±</mml:mo><mml:mn>0.007</mml:mn><mml:mfenced><mml:mi>syst</mml:mi></mml:mfenced></mml:math> . These results are in agreement with, and more precise than, previous determinations. They are used to constrain angles of the unitarity triangle, | sin (2 β + γ ) | and γ , to intervals that are consistent with the current world-average values.