I need to set the FieldB mandatory in transition, if the value of FiledA is set to YES.

[Suresh Ganapathy]

 I need to set the Field B mandatory in one of transition, if the value of Filed A is set to YES. Can this be achived by Validator ?

[Matthias Sohn]

on prem: possible, cloud : if u can declare a jira expression - i doubt 

but anyway, this would not be an elegant method, since the exception only follows after the attempted execution of the transition
you would have to make the transition with all inputs from the beginning again 

It is much more elegant to use behaviors (Scriptrunner addon). it supports transition screens.
with this behaviour-feature the transition can only be started when all conditions are met

[Maciej Dudziak _Forgappify_]

Hi @Suresh Ganapathy

Welcome, can you share what is the type of your project, is it team or company managed, and if you are on Jira Cloud or on premise?

Cheers

[Judah]

The easiest way to do this is with the 3rd party add-on ScriptRunner. They offer a solution called a "Behaviour" which is the easiest way to create this type of rule for a Jira field.