Last year the regulator opened up its register to platforms, several of which are believed to be in process of registering.
In June, the Fundraising Regulator updated the code to include a section specifically aimed at online fundraising platforms, which requires them to clearly explain their charges and offer guidance to people who set up fundraising pages.
Gerald Oppenheim, chief executive of the Fundraising Regulator, said: "The code was created to ensure that the public can have trust in our sector. With online donations continuing to grow, it is more important than ever that platforms such as JustGiving commit to upholding that code.
"We look forward to working closely with JustGiving to do just that."
In the statement announcing its registration, JustGiving said it had been carrying out a consultation with its charity partners since November to discuss "how to best work together in an environment of increasing public and governmental scrutiny".
It said it planned to release the outcomes of this consultation in the coming months.
JustGiving has faced criticism from MPs and in the national press over the amount of money it received in fees on donations to help the victims of terror attacks or disasters.
Keith Williams, general manager of JustGiving in the UK and Ireland, said: "With the changing and deepening consumer expectations around trust and consumer experience, this will be a renewed commitment from JustGiving to our charity partners to work closely, collaboratively and openly to ensure we are providing clear guidance to donors, fundraisers and charity partners alike."