Seyler & Tillemann was a Hamburg merchant bank in the 1750s and 1760s, that was owned by Abel Seyler and Johann Martin Tillemann. It involved itself in the currency market and "malicious" speculation with financial instruments during the Seven Years' War. It had ties to the brothers De Neufville in Amsterdam. Seyler & Tillemann went bankrupt as a result of the Amsterdam banking crisis of 1763 with 3–4 million Mark Banco in debts, an enormous sum. Its downfall also contributed to the downfall of other banking houses e.g. in Scandinavia.
In 1761 Seyler & Tillemann, acting as agents of their close business associate Heinrich Carl von Schimmelmann, leased the mint factory in Rethwisch from the impoverished Frederick Charles, Duke of Schleswig-Holstein-Sonderburg-Plön, a member of a cadet branch of the Danish royal family, to produce debased coins in the final years of the Seven Years' War. Seyler and Tillemann also owned a large silver refinery in Hamburg.