The firm gifted the Esher and Walton MP and former Brexit Secretary £44,258.99, as revealed in the latest Register of Members' Financial Interests.
The sum was described by Mr Raab as being for “the services of a member of staff for my Westminster office for six months”.
The donation was received by the MP on 1 March.
Mr Raab is currently second favourite, with bookmakers, to succeed Theresa May as leader of the Conservative Party, behind former Foreign Secretary and fellow Brexiteer Boris Johnson.
The MP resigned from Mrs May's Cabinet in November over opposition to the Northern Ireland 'backstop' as detailed in the Prime Minister's EU Withdrawal Agreement.
In his resignation letter her wrote: “I cannot support an indefinite backstop arrangement where the EU holds a veto over our ability to exit.”
A spokesman for the firm told Financial Planning Today: “Arbuthnot doesn’t comment on such matters.”