Interestingly, with all the technical writing being performed in the US today, there is no one organization that is accredited as being the organization for technical writers, or who provides "official" technical writing certification. But do not misunderstand: being certified is critical for a person to gain credibility as a technical writer; however, there are many different ways for a technical writer to get certified. Most every technical writer course you'll ever take will supply a tech writer certification at the end of it. What matters is where that certification comes from.
Certainly the most favorable source of technical writing certifications is the wide array of colleges and universities. There are many reputable institutions of higher learning that offer technical writer certifications--usually this is through a module of courses or workshops in the English or Business Departments. Obviously the more reputable the college or university, the more reputable your technical writer certification will be.
There are also many online courses, workshops, and associations that offer certifications for technical writers. Be careful, though--some of these are low-grade courses, designed mainly to take your money. Make sure you research the different technical writing certification providers and determine the quality of their technical writing training and their reputation in the technical writing industry.
A good resource is the Society for Technical Communication (STC) , found at www.stc.org. The STC has around 20,000 members and is dedicated to advancing the arts and sciences of technical communication. It helps technical writers network, get jobs, get published, and find instruction and certifications.
The Internet has great directories of technical writing companies, services, and instructional courses that will give more information on technical writer certification: