A second-half drive – struck seven minutes after the restart and from 30 yards – was all that separated the two sides on a gusty evening at the Rush Green Stadium.
An incident where the visitors nearly took the lead preceded the opener, Omar Rowe hitting the post from an indirect free-kick.
Jake Sinclair threatened in the dying stages of the encounter after he wrong-footed a Hammers defender before slamming the ball at the opposition’s ‘keeper.
Martin Hunter drew positives from his side’s first-half showing in the face of strong winds but was disappointed to see such a pleasing streak fade after the break.
The coach told saintsfc.co.uk
: “In the first half, there were gale-force winds. Still, we were very disciplined. We had three good chances to score. We played well against the wind – that was pleasing.
“Of course, they (West Ham) played against the wind in the second half. We talked about our passing changing to suit the conditions and the defenders getting inside to cause problems.
“We were very erratic, particularly in our defending in the second half. We won’t be using the conditions as an excuse.”
The Under-21 boss felt the Hammers confirmed his fears that they’d be a very powerful side with an underlying technical aspect.
Hunter also reserved praise for substitute Josh Sims – a winger whose Under-18 performances have seen him included in the Development Squad recetly – and said he is working at full capacity as he looks to deal with the rigours of moving up.
“West Ham were very direct,” the coach affirmed. “They have some good individual players and showed some good passages of play in the second half.
“Josh Sims has obviously still got plenty to learn as he’s only young and is making a big step up, but he has worked very hard for this team.”
Southampton Under-21s: Chris Johns, Jason McCarthy, Jack Stephens (c), Jordan Turnbull, Matt Targett, Dominic Gape, Jake Flannigan, Jake Sinclair (Ryan Seager), Lloyd Isgrove (Josh Sims 60), Omar Rowe, Sam Gallagher. Unused substitutes: James White (GK), Matt Young, Joe Curtis.