The Italian, who impressed the team during his outing in last year's young driver test in Abu Dhabi, is set to attend all of the races in 2013 in his new role and is first in line to stand in for either of the regular drivers should they become unavailable during a race weekend.
His predecessor, Jerome d'Ambrosio, remains on the team's books as reserve driver.
"I'm really happy that the team want me to be with them and I'm excited about the future," said Valsecchi. "I hope that this is a really good start to a career in F1.
"My last year in GP2 in 2012 was something I really focused on and in the end I succeeded. Now, in Formula 1, my target will be a little different but my focus to do the very best job is still the same."
Valsecchi has previously tested for HRT in 2010 and Caterham, which was then still running under the Lotus name, a year later.
Although Lotus has no plans to run him during the 12 days of pre-season testing, which will be shared between race drivers Kimi Raikkonen and Romain Grosjean, Valsecchi is confident that his new role is a big step towards an F1 race seat in the future.
"I very much hope, step by step, to get into Formula 1 as a race driver and being here as third driver is as near as you can get," he said.
"It's a great opportunity here; if I do the best job I can this year then it will open up my chances for the future and we'll see if I'm good enough."
D'Ambrosio, who stood in for the banned Grosjean in last year's Italian Grand Prix, is expected to focus on factory-based work in the simulator. He will largely work on the 2014 car and is not scheduled to attend all of the races.
However, he will also remain an option for the team should one of its drivers not be able to race even though Valsecchi is first in line.
"I learned from last year that you need to be ready in order to step straight into a race seat, so my aim will be to keep focused and be prepared," said d'Ambrosio.
"Of course my ultimate aim is always to gain a race seat, however I know that by doing the best job I can here - and proving myself valuable to the team - I'm in a very good position should an opportunity arise."
Nicolas Prost also remains on the Lotus roster as a development driver.