The agency will work with the charity's in-house team to strengthen the brand at a regional and national level.
Send a Cow provides livestock and training to some of the poorest families in Africa, helping them farm their way out of poverty.
But it wants to raise awareness that the charity has a broader range of gifts than just cows, as well as stressing that it also supports the communities themselves.
Amazon has been brought in to make the charity stand out in what is becoming an increasingly competitive marketplace of 'ethical gifts'. It is also tasked with attracting new donors and supporters through mainstream consumer and online media.
Amazon PR MD Louise Morriss said: 'Send a Cow was the first organisation to launch an ethical gift scheme in the UK and it continues to innovate. The charity is developing policies to ensure sustainability. It has a meaningful partnership with communities in Africa and is building a fantastically accessible and creative brand, providing the basis for an exciting campaign.'
The 12-month account was won following a three-way pitch. It is the first time the charity has used an external agency.