Yes, it is probably the T&P valve leaking. When did you last test it? Can you see the end of the drain piping from the T&P valve?
Go to the nearest "big box" store and buy an new T&P valve along with some teflon tape if you do not already have some. This is because you may need to replace the valve after testing it. A new valve costs less than $10.
Assuming that the open end of the drain piping is in an area where a large flow of water will cause no harm, pull the handle on the T&P valve smartly to the full open position, let it blow a couple of seconds and then release it. If your drain pipe is located where a flow will cause damage then it needs to be relocated. For now you can put a five gallon bucket under it.
Wait for about ten minutes and see if the drain line is still dripping. If not you are okay. If it is dripping then wait another hour and check again. If it is still dripping then you need to replace the T&P valve.