Russian President Vladimir Putin will attend the ceremony at the Kremlin and deliver a speech, with separatist officials of the four Ukrainian regions in attendance.
Moscow will formally annex four Russia-occupied regions of Ukraine at a Kremlin ceremony on Friday, President Vladimir Putin's spokesperson has said.
"Tomorrow in the Georgian Hall of the Grand Kremlin Palace at 15:00 (1200 GMT) a signing ceremony will take place on the incorporation of the new territories into Russia," spokesperson Dmitry Peskov said on Thursday.
He added that the Russian leader will make a major speech at the event.
Ukraine's Luhansk, Donetsk, Kherson and Zaporizhzhia regions are occupied by the Russian army, which Putin sent over the border on February 24.
Moscow organised what it called referendums in the four regions that it controls, with Moscow-backed officials saying this week that residents backed joining Russia.
All four Moscow-backed officials of the regions said they were in Moscow and expecting a meeting with Putin.
The move comes eight years after Moscow annexed the Crimea peninsula from Ukraine and would mark a significant escalation in the conflict.
The West has warned Russia not to press ahead with the annexations, with the G7 saying it would "never recognise" the move.
Kiev has asked for more military aid as a response.
The European Union is ready to make the Kremlin pay a “hefty price” for the latest escalation of the conflict in Ukraine by announcing the annexation of four regions, an EU official said.
EU “will not accept these sham referendums organised by Russia in Ukraine and will never accept any annexation of territory or any land by Russia,” Dana Spinant, the European Commission’s deputy chief spokesperson, told a daily news briefing.