"The so-called referendum breached international law, the declaration of independence which the Russian president accepted yesterday was against international law, and the absorption into the Russian Federation is, in our firm opinion, also against international law," she said.
Merkel stressed that international organisations including the United Nations, the European Union and the Organisation for Security and Cooperation in Europe shared that view.
President Vladimir Putin earlier on Tuesday signed a treaty making Crimea part of Russia, in a historic redrawing of Russia's borders, while shrugging off strong international objections.
The German chancellor stressed that amid the East-West standoff "we will... continue to bank on dialogue, on talks."
"We want a good development especially in Ukraine, including proper preparations for the elections," she said. "But the differences of opinion concerning the Crimea will remain."