Europe's powerhouse expanded by 0.3 percent quarter-on-quarter between January and March, adjusting for price, seasonal and calendar effects, federal statistics authority Destatis said.
That was half the pace seen in the previous three months, and analysts surveyed by data company Factset had predicted growth of 0.4 percent.
Destatis highlighted "positive impulses" from the domestic economy, as firms increased investments in buildings and equipment and households slightly increased consumer spending.
But government outlays fell slightly for the first time in almost five years, slowing growth, while both imports and exports fell back over the quarter compared with October to December.
After an unexpectedly strong year for the 19-nation single currency area in 2017, Germany's weaker first-quarter growth follows a run of economic data suggesting a slowdown could be on the way for the eurozone.
Meanwhile, fears linger that the European Union could be headed for a trade showdown with US President Donald Trump.
"The German economy lost a lot of momentum in Q1," Oxford Economics analyst Oliver Rakau tweeted.
"Part of the weakness will be reversed as transitory factors fade, but we think there is more to it."
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