In the 1965–66 season West Ham were again in European football as holders of the Cup Winners' Cup. They also reached the 1966 League Cup Final. Again Byrne was on the scoresheet in the Cup Winners' Cup, in the second round against Olympiacos, the third round against Magedeburg and in the semi-final against Borussia Dortmund where they were eliminated from the competition. He scored five goals in six games in the League Cup including one in the first-leg of the final against West Bromwich Albion which West Ham won 2–1. Albion won the second leg, at The Hawthorns, 4–1 to take the trophy by an aggregate score of 5–3. Byrne finished the season with 17 goals in all competitions, someway behind Geoff Hurst, who, on the verge of his 1966 World Cup victory, scored 40 goals in 59 games.