Otto Wacker (?-?) was a German art dealer who became famous for commissioning and selling forgeries of paintings of Vincent Van Gogh.

Otto Wacker became an art dealer in 1925 after various false starts in other professions. He developed a reputation of reliability in the art field. Some of the fraudulent Van Goghs were the work of French painter Claude-Emile Schuffenecker.

Wacker managed to convince prominent Van Gogh experts Jacob Baart de la Faille , Meier-Graefe and Hans Rosenhagen that the paintings he was selling were genuine. The experts accepted his tall tale that the Russian owner had bought the paintings, transferred them to Switzerland illegally and had commissioned an illegal agent to sell them. Experts understood the need of this Russian to remain anonymous to prevent reprisals to relatives who still lived in Soviet Union and supplied certificates of authenticity without proof of provenance. Thannhauser, Matthiesen and Goldschmidt galleries bought some of the paintings.

Wacker's paintings were to be exhibited in December 1927 in Cassirer Exhibition by the firm of Paul Cassirer in Berlin. It was organized to coincide with the publication of de la Faille's standard catalogue of Van Gogh's work. When Wacker delivered the last four paintings, Grete Ring and Walter Feilchenfeldt , the general managers of the exhibition, noticed the differences and recognized them as fakes. The canvases were returned to Wacker.

Further investigation revealed 33 suspect paintings, all of them supplied by Wacker. Galleries that had sold his paintings asked their customers to return them. Hugo Peris, lawyer who had bought most of the paintings, still insisted that they were authentic. In December 1928 Matthiesen gallery, with the aid of the Federation of German Art and Antique Dealers, sued Wacker. De la Faille, who had contacted Van Gogh estate, changed his mind and listed the paintings in Les faux van Goghs (The False Van Goghs), published 1930.

Trial against Wacker begun in April 6 1932. Vincent Wilhelm van Gogh, nephew of the painter, gave the first evidence at the trial and stated that family records did not include any Russian who would have purchased any paintings. De La Faille, on the other hand, had changed his mind once again and claimed that at least 5 of the paintings were genuine.

During the trial, various experts did not come to full agreement of what paintings were authentic (and the argument was to continue in some circles for years afterwards). Meier-Grafe admitted his mistake and even that the expert opinion could be fallible. Hans Rosenhagen said that 14 of the works were inferior but genuine.

However, Dutch painter A M de Wild found that the pigments used in the paintings were not similar to those Van Gogh had used. Art restorer Kurt Wehite showed with X-rays that the painting techniques were different (although he used a painting that was declared a forgery in the 1970s). Later it was found that the paintings had been not painted on French canvases at all.

On April 19 1932 Wacker for sentenced for fraud, after appeal, for 19 months in prison and to a fine of DM30.000.

Also some former directors of the Bank für Deutsche Beamte , who had been speculating in the paintings in the behalf of the bank, were sued.

After the World War II, Otto Wacker lived in East Berlin. He had abandoned the art market. His paintings have since been disappeared but Van Gogh experts still cannot agree about some of them.