na-adjective
Describes a person, attitude, or decision based on reason rather than emotion. Often used in contrast with 感情的 (emotional).
彼はいつも理性的な判断をする。
He always makes rational decisions.
感情的にならず、理性的に話し合おう。
Let's discuss this rationally, without getting emotional.
合理的 means 'rational' in the sense of efficient or logical, often applied to systems or methods. 理性的 is more about a person's capacity for reason and self-control.
Compound of 理性 (reason) + 的 (-like, -ical). 理性 is from Chinese 理性 (lǐxìng), and 的 is a Sino-Japanese suffix forming adjectival nouns.