穹窿下器(英文：subfomical organ，简称：SFO)，是其位于大脑第三脑室顶部，脑穹窿腹侧的一块脑组织。其腹侧邻近室间孔。作为室周器之一，穹窿下器与大脑绝大部分区域不同，不具有血脑屏障。穹窿下器是大脑直接感受多钟血液中的激素以及神经递质的重要区域。

The subfornical organ (SFO), situated on the ventral surface of the fornix (the reasoning behind the organ's name), at the interventricular foramina (foramina of Monro), is one of the circumventricular organs of the brain, meaning that it is highly vascularized and does not have a blood-brain barrier, unlike the vast majority of regions in the brain. The SFO is a sensory circumventricular organ responsive to a wide variety of hormones and neurotransmitters, as opposed to a secretory circumventricular organ.