Mr Gruffydd pledged to "reflect, reform and renew" the party at "a critical juncture" for Plaid Cymru following a damning review into a "toxic culture" and failures by the leadership to deal with complaints of bullying, misogyny and sexual harassment.
He was unanimously chosen for the role by his fellow Senedd members in a meeting on Thursday morning.
That decision was ratified on Saturday by the party's National Council, which represents the wider membership, at a meeting in Aberystwyth. Mr Gruffydd will formally take over from Mr Price on Wednesday 17 May.
Speaking after today's ratification, he said: "It is an immense honour for me to be entrusted with the responsibility of leading Plaid Cymru until a new leader is in post in the summer.
"Despite it being short, my tenure as the interim leader of Plaid Cymru comes at a critical juncture for the party. We’ve been reflecting, we are reforming, and we will renew our mission in light of the findings of Project Pawb.
"Its findings run counter to our core beliefs and values. In our period of reflection we remind ourselves of what Plaid Cymru aspires to be – an inclusive party, a party that values its staff, a party founded on the best principles of progressive action – with fairness and equality embedded in its DNA.
"Accelerating the pace of reform will be the priority for the political, professional, and voluntary wing of the party. By doing this we can renew our purpose, delivering the key aspects of our manifesto through the co-operation agreement, offering solutions when Wales' interests aren’t being served and working harder than ever to protect our communities.
"Moving forward, united, we will put down new and stronger foundations with our ambition undimmed."
Adam Price resigned as leader late on Wednesday night following the Prosiect Pawb review by former Plaid Cymru AM Nerys Evans.
He had initially intended to continue in post in order to implement the recommendations of the report but some members of the Plaid Cymru Senedd group opposed that move.
Following a late night meeting with the party’s other governing body, the National Executive Committee, Mr Price announced that he would quit after five years in the top job.
What happens next?
Adam Price is expected to resign formally on Tuesday after he's taken part in First Minister's Questions for the last time as leader.
There are other formalities to go through such as transferring staff contracts and notifying the Electoral Commission.
After that, on Wednesday, Llŷr Gruffydd will be interim leader until a permanent leader is chosen by the party.
The leadership contest is expected to start straight away, with nominations due to close on 16th June and a result due "in the summer".
In 2018 the timetable was quite short, with only three weeks elapsing between the first hustings and the final announcement.
Plaid Cymru's constitution states that the party's leader has to be a member of the group in the Welsh Parliament, ruling out MPs, councillors and other senior figures.
A rule change means that whoever is interim leader cannot stand and another senior figure, the Ceredigion MS Elin Jones has also ruled herself out, saying that she intends to continue as the Senedd's Llywydd.
With Ms Jones out of the contest along with Mr Price and Mr Gruffydd and another MS, Rhys ab Owen suspended while an investigation by the Standards Commissioner takes place, that leaves nine possible candidates for the top job:
Mabon ap Gwynfor
Rhun ap Iorwerth
Peredur Owen Griffiths
Ynys Môn MS Rhun ap Iorwerth came second in 2018 but has said that he intends to fight for the island's Westminster seat which would bar him from becoming leader.
Elin Jones has also called for the party to unite around a single candidate to avoid what she called a "divisive, distracting leadership election".
However, others think that Plaid Cymru needs to have a proper discussion about its future if it's to make a new start.