Aaron Saucedo, 23, had already been charged with murder in connection with a killing in August 2015, police say.
Now he has been linked to the deaths of eight other people, between January and July 2016.
Police say they have not identified a motive but believe he acted alone. His lawyer has not commented.
The 2015 victim was a 61-year-old man who had dated the suspect's mother, Phoenix Police Chief Jeri Williams said.
The other victims, including a 12-year-old girl and a 55-year-old woman, seem to have been targeted at random.
"Today is a good day in the city of Phoenix," Mayor Greg Stanton said.
Police gave few details on how they linked the man to the killings, which stopped last July amid extensive media coverage.
But they said the evidence included surveillance video, witness statements and ballistics material.
He faces 26 additional felony counts, including multiple counts of homicide, aggravated assault and drive-by shootings.
The Maricopa County attorney has not yet decided which charges to formally bring against him.

The 31-year-old had publicly revealed she was expecting her third child during a concert in May.
A statement on her Twitter account on Monday said: "Charlotte and Jonny are very sad to announce that they lost their baby. Now is a time for grieving and being together as a family.
"We kindly ask everyone to respect that peace."
The Cardiff-born star and her partner, musician Jonny Powell, have been together for seven years.
She is already a mother to nine-year-old Ruby and Dexter, eight, from her previous relationship with Gavin Henson.

Tall and like football? Go and play centre-back. Enjoy rugby union and have a bit of pace? Winger.
Step forward Somerset's Dom Bess, who took 6-28 on his County Championship debut on Tuesday.
How did he come to bowl off-spin?
"I was always a little porker when I was younger, so I never took the long run-up," he said.
"I always took five or six steps and just trotted in."
Bess now has the best figures by a Somerset debutant for 55 years, since Tony Pearson returned 7-63 against Worcestershire in 1961, according to the Guardian.
But any 'puppy fat' the 19-year-old may have had has long gone, with the help of strength and conditioning coach Darren Veness.
"I met Daz and he made me run a lot, which I'm grateful for now," Bess told BBC Radio Bristol.
"Ask Daz about me, I was always a porker."
Just a month ago Bess, from Sidmouth, was taking 6-65 for Devon in the Minor Counties Championship as they lost to bottom-of-the-table Oxfordshire.
At Taunton, however, his victims included Ian Bell and Jonathan Trott, who have played a combined total of 170 Tests for England.
His family have a strong presence in the sport, with cousins Zac and Josh also representing Devon, but who was at the County Ground to watch his moment of glory?
"Dad was on holiday and mum was working, so I just had my girlfriend, her parents and a couple of my mates (watching)," he said.
"I think dad will be a bit disappointed - he's gone to Greece, somewhere like that, for a week away."
Police in the US state of Arizona say a man arrested last month is responsible for a series of deadly shootings in Phoenix last year.
[NEXT_CONCEPT]
Singer Charlotte Church has lost her unborn baby, she has announced.
[NEXT_CONCEPT]
Whether it comes down to skill level, body shape, or pure accident, anyone who has played a sport will have some memory of how they came to be given a particular role.