In its accounts for the year to 31 December 2018, the charity brought in £142m, up from £140.9m the year before.
The accounts show that fundraising income accounted for the bulk of the increase, with legacies rising by £4m to take the total raised through donations and legacies to £122.6m
But expenditure increased substantially, hitting £159.8m for the year, compared with £129.4m previously.
An actuarial gain on the charity’s defined-benefit pension scheme of £18.1m meant the charity ended the year with a deficit of £6.9m.
The charity had made a surplus of £24.8m the year before, according to the accounts.
Legacy income was at £81.4m, while donations remained flat at £41.2m, compared with £41.9m in 2017.
The accounts say that the slight reduction in income from other forms of fundraising such as donations, excluding legacies, reflected new regulatory constraints and the cost of compliance.
Paul Draycott, chair of the RSPCA, said in his introduction to the accounts: "I am excited about the prospects for the RSPCA in 2019 as we continue to modernise and transform our organisation.
"We must also be ready to adapt, innovate and collaborate with others to continue to advance our mission and vision.
"If we succeed, we will have created a society that is fit for purpose into the future and best placed to do even more for the animals and the people in the communities we serve."
The accounts also show that the highest earner at the RSPCA had a total annual salary and benefits of between £220,000 and £229,999. The previous year’s accounts showed a highest pay packet of between £190,000 and £199,999.
Chris Sherwood was appointed as the charity’s new chief executive last year, and has subsequently announced governance reforms to the charity, including halving the size of the charity’s board.