The paper reports on gas-puff experiments performed for the first time with the large plasma-focus POSEIDON facility operated at 130 to 206 kJ. Different diagnostic techniques have been used to observe interaction of a deuterium plasma current sheath with deuterium or argon targets which were produced by a fast-acting gas valve at the end of the inner electrode. It has been shown that POSEIDON can be operated with gas puffing in a controlled way.