Siegfried Cremer was also extraordinarily active as a collector. The development of an extensive art collection of the European avant-garde of the 1950s and 1960s with Zero, Kinetics, Nouveau réalisme, Fluxus, Pop Art and concrete poetry, which began in Krefeld around 1957, was finally completed in 1974. Today, part of the collection is housed as the Cremer Collection Foundation in the Westphalian State Museum in Münster, while another part is owned by the Museum am Ostwall in the Dortmunder U in Dortmund. In Düsseldorf, Cremer builds up a collection of nomadic carpets from the end of 1989, which is completed in 1993.
From the mid-1990s, a collection of vases for individual flowers made of utility glass is created, which goes as a donation to the Westphalian State Museum of Industrial Culture Glashütte Gernheim in Petershagen. The collections of candlesticks and crosses begun thereafter remained unfinished.