The election of Rwanda was likely to renew questions about the image of the council, as it tries to overcome division and find a way to end the war in Syria.
An unpublished UN experts' report, leaked to the media this week, accuses Rwanda and Uganda of actively supporting the M23 rebels in eastern DRC. Rwanda and Uganda deny the charges.
The M23's rebellion has caused more than 200 000 villagers in the province of North Kivu to flee their homes this year. Eastern DRC has been engulfed in fighting since the 1994 Rwanda genocide.
Rwanda won a two-year non-permanent seat on the council, starting in 2013. It prevailed despite a July report by the UN experts panel that accused senior Rwandan security officials of supporting the rebellion and sending arms into DRC.
Ida Sawyer, a DRC-based Human Rights Watch researcher, said Rwanda has been rewarded after violating the Security Council's arms embargo on DRC and "undermining the work of the UN by propping up the abusive M23 rebels."
"Kigali is now in a position to try to shield its own officials implicated in abuses from the UN sanctions, which is a flagrant conflict of interest," she said. "Other Security Council members now have an even greater responsibility to hold Rwanda to account."
Rwanda's Foreign Minister Louise Mushikiwabo rejected the claims in the latest UN report and thanked the assembly for voting it onto the council for the first time since its genocide.
Wave of violence
"The contrast could not be sharper between that previous tenure — when a genocidal government occupied a prized Security Council seat as its agents waged genocide back home — and the Rwanda of today: a nation of peace, unity, progress and optimism," Mushikiwabo said.
DRC opposed Rwanda's bid.
Rwanda was welcomed to the council by Britain's deputy UN representative, Philip Parham, who said it will "bring to the Council the particular perspective of a country that has overcome serious conflict and has done so more successfully than many," he said.
Eastern DRC’s latest wave of violence flared earlier this year when M23 rebels linked to General Bosco Ntaganda claimed that they weren't being paid by the Congolese military and that the government had failed to hold up their end of the 2009 peace deal that integrated them into the army. Ntaganda is wanted for war crimes by the International Criminal Court.
Other nations joining the Security Council in 2013 include Argentina, Australia, Luxembourg and South Korea. Each gained the required two-thirds majority in the General Assembly.
Australia joins after intense lobbying by Prime Minister Julia Gillard during last month's gathering of world leaders at the UN. The bid earned criticism from the Australian opposition, which claimed it was a wasteful distraction.
Luxembourg, a founding member of the UN, will join the council for the first time. Luxembourg and Australia beat out Finland for spot.
Argentina will be on the council for the eighth time in its history after receiving more votes than any other candidate country.
South Korea gained a seat that was also sought by Bhutan and Cambodia. Seoul's foreign ministry said in a statement that it would use its platform on the council to take a lead role in managing the situation in North Korea.