Mark Hallam, who worked at the charity from 2005 until last year, is now director of finance, IT and trading at the Marine Society and Sea Cadets.
He contends that Paul Boissier, chief executive of the charity, and some board members made his position untenable. Boissier took up the post of chief executive of the RNLI in 2009 after retiring from a 35-year career in the Navy.
An RNLI spokeswoman said: "The RNLI would like to reassure people that any allegation brought to our attention by our staff or volunteers is thoroughly investigated and fairly dealt with.
"Employment tribunal proceedings relating to claims of unfair dismissal are being defended and, as such, it is not appropriate for us to comment further."
The hearing is scheduled for 5 and 6 December.