The Liberal group joined the so-called 'stable majority' agreement with parliament's two main political groups late last week.
That agreement, primarily a pact to ensure a pro-European grand coalition within the assembly, will give the three groups a comfortable working majority of around 100 MEPs in the new parliament.
The original deal combining the EPP's 221 MEPs and the S&D's 191 deputies, although providing a small majority, fell short of providing a sustainable and secure working margin for the length of the parliament.
The inclusion of the ALDE group's 67 deputies, should in theory provide a large enough margin against any losses from the three main groups during the next five years.
The Liberal group have been campaigning since the European elections in May to highlight that the support of their members was essential to the creation of a pro-European working majority in the parliament.
An ALDE group source told the Parliament Magazine that, "the message seems to have got home [as] the two big groups conceded that they needed to bolster their flimsy majority by including us".
One of the key aspects of the original deal, which saw the Socialists formally back the EPP's candidate for commission president Jean-Claude Juncker, is the continuation of the split mandate agreement between the two biggest groups on the post of parliament president.
Former parliament president, Martin Schulz, who failed in his bid to become the new commission president, is now set to be anointed in the post again later this week in Strasbourg.
With the support of the three groups now assured, Schulz's bid is likely to be rubber-stamped at first vote.
"The agreement of 24 June between the EPP and S&D groups, relating to ongoing discussions on key EP positions, will now be extended to the ALDE Group, ensuring a broader base for building a stable majority for the next European commission" - ALDE group
The three-party grand coalition agreement will also help secure other backroom deals on key parliament posts for the three groups, although social media reports that the price of obtaining the ALDE group's votes would be a shoo-in for Guy Verhofstadt as parliament president in the second half of the assembly's term were hastily dismissed.
In a short statement, the ALDE group said, "The agreement of 24 June between the EPP and S&D groups, relating to ongoing discussions on key EP positions, will now be extended to the ALDE group, ensuring a broader base for building a stable majority for the next European commission."
The deal also effectively provides a European parliament majority for commission president-elect Jean-Claude Juncker, who was nominated on Friday as the new commission chief by all but two EU member states.
Juncker will seek to secure parliament's backing during the second Strasbourg session on 14-17 July.