State Councillor and Foreign Minister Wang Yi and Solomon Islands Foreign Minister Jeremiah Manele, and foreign ministry spokesman Wang Wenbin told reporters the deal had been signed but did not give details of when or where the signing took place.
It is also not known what the deal entails.
Canberra is concerned the deal could be a step towards a Chinese military presence less than 2000 kilometres from Australia.
Solomon Islands' Prime Minister, Manasseh Sogavare, was asked by Australia's Minister for International Development and the Pacific Zed Seselja not to sign the deal during a visit last week.
Together with the foreign minister, Senator Seselja issued a statement on Tuesday evening outlining Australia's "deep disappointment" with the signing of the cooperation agreement.
"We respect Solomon Islands' right to make sovereign decisions about its national security," the senators wrote.
"Our consistently stated view, including from the perspective of Australia's national interests, remains that the Pacific family is best placed to meet the security needs of the region."
On Monday, the White House said a high-level US delegation including Indo-Pacific coordinator Kurt Campbell would travel to Honiara this week to discuss concerns about China and the reopening of a US embassy.
Chinese spokesperson Wang was critical of the visits by Australia and the U.S.
"Deliberate attempts to inflate tensions and mobilise rival camps are also doomed to fail," Wang said on Tuesday.
Douglas Ete, Solomon Islands chairman of the public accounts committee, told the country's parliament that Chinese officials would arrive next month.
Ete said the visit meant the two nations would increase cooperation on trade, education and fisheries but added he rejected the idea of the Solomons signing a security pact with China to set up a military base.