Short answer: Not. You can call yourself one right now, if you wanted to.
Longer, less irritating answer:
The fitness certification industry is regulated, requiring a test to prove you know the basics. Group fitness instructors and personal trainers generally have to be certified to get hired for any type of real job, but it isn’t illegal to call yourself a personal trainer and accept money in exchange for telling someone to do pushups. You will just have to find clients yourself because no gym or studio will hire you and you’ll probably get sued when you kill someone and you won’t have insurance to protect you because you aren’t certified.
Yoga teachers, as it is a relatively new career, have even more lax standards. There is no requirement to be certified, and if you know what you’re talking about, a lot of places will hire you without one.
That being said, if you want to have any type of real career in the field and any credibility with clients whatsoever, you will an accredited program behind you, which for yoga is Yoga Alliance. In 1999, Yoga Alliance established a national Yoga Teachers’ Registry to recognize and promote yoga teachers with training that meets their minimum standards. If teachers meet these standards, they are eligible to register as Registered Yoga Teachers.
Here are the Registered Yoga Teacher standards.
If this isn’t your route, a lot of yoga studios offer teacher training courses, too, so ask around to different studios in your area and see if they offer classes.