Russia is to cut the price of gas it sells to Ukraine and is offering to buy $US15 billion in government bonds. The gas discount is almost a third of the price.
Presidents Vladimir Putin and Viktor Yanukovych annnounced the moves in Moscow on Tuesday, but said they had not discussed Ukraine joining a Russian customs union.
The announcement was made as protests continued in Kiev over deepening ties with Russia.
TheBBC reports the price of Russian gas supplied to Ukraine has been reduced from more than $US400 per 1000 cubic metres to $268.5.
Mr Putin said the assistance was not "tied to any conditions".
However, a BBC correspondent says the two signed a number of agreements on economic and industrial co-operation.
A U-turn by Mr Yanukovych on an EU association agreement deal last month sparked mass demonstrations.
He has admitted his decision had been influenced by heavy pressure from Russia.
The protests are the largest since the Orange Revolution in Ukraine in 2004.
Opposition leaders in Ukraine have denounced the agreement.
"He has given up Ukraine's national interests, given up independence," said Vitaly Klitschko, a party leader and a heavyweight boxing champion.