The 1991 German Open was a men's tennis tournament played on outdoor clay courts. It was the 84th edition of the Hamburg Masters (Hamburg Masters), and was part of the ATP Super 9 of the 1991 ATP Tour. It took place at the Rothenbaum Tennis Center in Hamburg, Germany, from May 6 through May 13, 1991. Karel Nováček won the singles title.
Karel Nováček defeated Magnus Gustafsson, 6–3, 6–3, 5–7, 0–6, 6–1 
- It was Karel Nováček's 2nd title of the year, and his 5th overall. It was his 1st Masters title of the year, and overall.
Sergio Casal / Emilio Sánchez defeated Cassio Motta / Danie Visser, 7–6, 7–6