The person spoke on condition of anonymity Friday because the deal had not been officially announced.
Iannetta, an 11-year major league veteran, hit .210 with seven home runs in 94 games with the Seattle Mariners last season.
Iannetta will compete with Chris Herrmann and Jeff Mathis for playing time. The 33-year-old played six seasons with Colorado and four with the Los Angeles Angels before going to Seattle a year ago. He is a career .229 hitter with 107 home runs.
The Diamondbacks also avoided arbitration on Friday with left-handed pitcher Patrick Corbin, shortstop Chris Owings, Herrmann and right-handed reliever Randall Delgado by agreeing to one-year contracts.
Corbin agreed to $3.95 million after making $2,525,000 last season. He was coming off Tommy John surgery last year and was slotted to be a strong No. 3 pitcher in the rotation. But he was ineffective most of the time and eventually was sent to the bullpen. However, he looked stronger in his late-season appearances.
Owings agreed to $2.3 million in his first season of arbitration eligibility. He was used some in center field last season due to injuries at the position. He hit .277, stole 21 bases and was tied for the major league lead in triples with 11 while making $530,500.
Miller has asked for $5.1 million in arbitration after making $4.35 million during a disastrous first season with Arizona. Acquired from Atlanta last offseason in a deal that sent No. 1 overall draft pick Dansby Swanson to the Braves, Miller went 3-12 with a 6.15 ERA and was demoted to the minor leagues in July. The Diamondbacks have offered him $4.7 million.
Walker was acquired from Seattle this offseason in a deal that sent shortstop Jean Segura to the Mariners. He's asked for $2.6 million and Arizona has offered $2.25 million in Walker's first year of arbitration eligibility. The 24-year-old made $528,600 last season while going 8-11 with a 4.22 ERA.
Delgado agreed to a $1,775,000 contract after making $1,275,000 last season. Delgado appeared in 79 games last year out of the bullpen, going 5-2 with a 4.44 ERA. The right-hander has pitched 147 innings over the last two seasons almost entirely out of the bullpen.
Herrmann will make $937,500 on a one-year deal after making $516,900 last year. He hit .284 with six homers while playing in 56 games with the Diamondbacks in 2016.