The Latvia international, 24, made just three appearances in all competitions last season for the Royals in an injury-affected campaign.
He could make his competitive debut for Lech in their Europa League qualifier against Macedonian club Pelister on Thursday.
Rakels joined Reading in January 2016 from Polish club Cracovia.
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Reading forward Deniss Rakels has agreed to join Lech Poznan in Poland on a season-long loan deal.