Steinmeier spoke during his first visit as president to Poland, which will become the EU's fifth largest member by population following Britain's departure from the bloc.
Steinmeier had visited his nation's eastern neighbor several times in his earlier position of foreign minister.
The one-day visit comes as Poland's ties have cooled with Germany, as well with the rest of the EU, under a nationalist Polish government that has at times has taken Euroskeptic positions.
"The crisis did not start with Brexit but without a doubt it has gotten deeper with Brexit and that makes it even more important that the 27 of us stick together and we need Poland for that."
He said he also hoped that Poland could reach an understanding with the European Commission, which is concerned over rule of law in the country due to the government's erosion of independence of the Constitutional Tribunal.
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