I was unable to wake up for suhoor and as soon I woke up, I realized it was already Fajr time. I quickly took a sip of water since I missed Suhoor. Is my fast still valid?
Fasting starts at the time of Fajr(true dawn). If you overslept, you're not exempted to take a few moments to eat/drink. You missed the opening window for Suhoor so you'd have to fast without eating anything prior.
Since you woke up and took a sip of water after missing Suhoor, you've already broken your fast.
It was narrated by Ibn Khuzaymah (1986) and Ibn Hibbaan (7491) from Abu Umaamah al-Baahili who said:
I heard the Messenger of Allah (SAWS) say: "Whilst I was sleeping two men came to me and took me by the arm and brought me to a cragged mountain. They said, ‘Climb up.’ I said, ‘I cannot.’ They said, ‘We will make it easy for you.’ So I climbed up until I was at the top of the mountain. Then I heard loud voices. I said, ‘What are these voices?’ They said, ‘This is the howling of the people of Hell.” Then I was taken until I saw people hanging by their hamstrings, with the sides of their mouths torn and blood pouring from their mouths.’ I said, ‘Who are these?’ He said, ‘These are people who broke their fast before it was time.’"
-Classed as saheeh by al-Albaani in Saheeh Mawaarid al-Zam’aan, no. 1509.
Al-Albaani (may Allah have mercy on him) said:
"This is the punishment of those who fasted then broke the fast deliberately before the time for breaking the fast came, so who about those who do not fast at all? We ask Allah to keep us safe and sound in this world and in the Hereafter."
If you unintentionally ate, then this does not invalidate fast and if you were unaware of the ruling I am not too sure.