diff --git a/sci-physics/pythia/pythia-8.3.16-r1.ebuild b/sci-physics/pythia/pythia-8.3.16-r1.ebuild
new file mode 100644
index 0000000000000..cce0c6485d549
--- /dev/null
+++ b/sci-physics/pythia/pythia-8.3.16-r1.ebuild
@@ -0,0 +1,216 @@
+# Copyright 1999-2025 Gentoo Authors
+# Distributed under the terms of the GNU General Public License v2
+
+EAPI=8
+
+PYTHON_COMPAT=( python3_{11..13} )
+inherit toolchain-funcs python-single-r1 optfeature
+
+MY_PV=$(ver_cut 1-2)
+MY_P="${PN}${PV//./}"
+LHA_VER="6.2.1"
+
+DESCRIPTION="Lund Monte Carlo high-energy physics event generator"
+HOMEPAGE="https://pythia.org/"
+SRC_URI="test? ( lhapdf? (
+		https://lhapdfsets.web.cern.ch/lhapdfsets/current/CT10.tar.gz
+		https://lhapdfsets.web.cern.ch/lhapdfsets/current/MRST2007lomod.tar.gz
+		https://lhapdfsets.web.cern.ch/lhapdfsets/current/NNPDF23_nlo_as_0119_qed_mc.tar.gz
+		https://lhapdfsets.web.cern.ch/lhapdfsets/current/NNPDF23_nnlo_as_0119_qed_mc.tar.gz
+		https://lhapdfsets.web.cern.ch/lhapdfsets/current/cteq66.tar.gz
+		https://lhapdfsets.web.cern.ch/lhapdfsets/current/cteq6l1.tar.gz
+		https://www.hepforge.org/downloads/lhapdf/pdfsets/v6.backup/${LHA_VER}/MRST2004qed.tar.gz
+	) )
+"
+if [[ ${PV} == 9999 ]]; then
+	inherit git-r3
+	EGIT_REPO_URI="https://gitlab.com/Pythia8/releases"
+else
+	SRC_URI="https://pythia.org/download/${PN}${MY_PV//./}/${MY_P}.tgz
+	${SRC_URI}"
+	KEYWORDS="~amd64 ~x86"
+	S="${WORKDIR}/${MY_P}"
+fi
+
+LICENSE="GPL-2"
+SLOT="8"
+IUSE="doc examples fastjet +hepmc3 lhapdf root test zlib python highfive mpich rivet static-libs" # evtgen mg5mes rivet powheg
+RESTRICT="!test? ( test )"
+REQUIRED_USE="python? ( ${PYTHON_REQUIRED_USE} )"
+
+RDEPEND="
+	fastjet? ( sci-physics/fastjet )
+	hepmc3? ( sci-physics/hepmc:3= )
+	lhapdf? ( sci-physics/lhapdf:= )
+	zlib? ( virtual/zlib:= )
+	highfive? (
+		sci-libs/highfive
+		sci-libs/hdf5[cxx]
+	)
+	rivet? ( >=sci-physics/rivet-4:* )
+	mpich? ( sys-cluster/mpich )
+	python? ( ${PYTHON_DEPS} )
+"
+DEPEND="${RDEPEND}"
+# ROOT is used only when building related tests
+BDEPEND="
+	test? (
+		root? ( sci-physics/root:= )
+	)
+"
+
+PATCHES=(
+	"${FILESDIR}"/${PN}-8.3.15-ar.patch
+)
+
+pkg_setup() {
+	use python && python-single-r1_pkg_setup
+}
+
+pkg_pretend() {
+	if use root && ! use test; then
+		ewarn "ROOT support will only affect examples code build during test stage."
+		ewarn "Since you have tests disabled, this is a no-op."
+	fi
+}
+
+src_prepare() {
+	PYTHIADIR="/usr/share/Pythia8"
+	EPYTHIADIR="${EPREFIX}${PYTHIADIR}"
+
+	default
+	# set datadir for xmldor in include file
+	sed -i \
+		-e "s|../share/Pythia8/xmldoc|${EPYTHIADIR}/xmldoc|" \
+		include/Pythia8/Pythia.h || die
+	# respect libdir, prefix, flags
+	sed -i \
+		-e "s|/lib|/$(get_libdir)|g" \
+		-e "s|/usr|${EPREFIX}/usr|g" \
+		-e "s|-O2|${CXXFLAGS}|g" \
+		-e "s|Cint|Core|g" \
+		configure || die
+	# we use lhapdf6 instead of lhapdf5
+	# some PDFs changed, use something similar
+	sed -i \
+		-e "s|LHAPDF5|LHAPDF6|g" \
+		-e "s|\.LHgrid||g" \
+		-e "s|\.LHpdf||g" \
+		-e "s|MRST2001lo|MRST2007lomod|g" \
+		-e "s|cteq6ll|cteq6l1|g" \
+		-e "s|cteq6m|cteq66|g" \
+		examples/*.{cc,cmnd} || die
+	# After lhapdf5->6 migration PDFs are identical within ~1/1000
+	# https|//www.hepforge.org/archive/lhapdf/pdfsets/6.1/README
+	sed -i \
+		-e "s|1e-8|3e-1|g" \
+		-e "s|nlo_as_0119_qed|nlo_as_0119_qed_mc|g" \
+		-e "s|xmldoc|share/Pythia8/xmldoc|g" \
+		examples/main203.cc || die
+	# ask cflags from root
+	sed -i "s|root-config|root-config --cflags|g" examples/Makefile || die
+
+	sed -i 's|libpythia8\.a|libpythia8$(LIB_SUFFIX)|g' \
+		examples/Makefile || die
+}
+
+# TODO: the following optional packages are out of Gentoo tree:
+# - EvtGen http://atlas-computing.web.cern.ch/atlas-computing/links/externalDirectory/EvtGen/
+# - PowHEG http://powhegbox.mib.infn.it/
+# - ProMC  https://github.com/Argonne-National-Laboratory/ProMC/
+src_configure() {
+	# homemade configure script
+	local -x CXX="$(tc-getCXX) ${CXXFLAGS} ${LDFLAGS}"
+	./configure \
+		--arch=Linux \
+		--prefix="${EPREFIX}/usr" \
+		--prefix-lib="${EPREFIX}/usr/$(get_libdir)" \
+		--prefix-share="${EPYTHIADIR}" \
+		$(usex fastjet "--with-fastjet3" "") \
+		$(usex zlib "--with-gzip" "") \
+		$(use_with hepmc3) \
+		$(use_with highfive) \
+		$(usex highfive --with-hdf5 "") \
+		$(use_with python) \
+		$(use_with rivet) \
+		$(use_with mpich) \
+		$(use_with hepmc2) \
+		$(usex lhapdf "--with-lhapdf6
+			--with-lhapdf6-plugin=LHAPDF6.h
+			--with-lhapdf6-lib=${EPREFIX}/usr/$(get_libdir)" "") \
+		$(usex root "--with-root
+			--with-root-include=${EPREFIX}/usr/include/root
+			--with-root-lib=${EPREFIX}/usr/$(get_libdir)/root" "") \
+		|| die
+
+	# fix pythia config script
+	sed -i \
+		-e 's|Pythia8/examples/Makefile.inc|Pythia8/Makefile.inc|' \
+		-e "s|CFG_FILE=.*|CFG_FILE=${EPYTHIADIR}/Makefile.inc|" \
+		-e 's|LINE%=|LINE%%=|' \
+		bin/pythia8-config || die
+}
+
+src_test() {
+	cd examples || die
+
+	local tests="$(echo main{{101..103},{121..127}})"
+	use hepmc3 && tests+=" $(echo main{131..135})"
+	use hepmc3 && use mpich && use highfive && tests+=" $(echo main136)"
+	use lhapdf && tests+=" $(echo main{201..204})"
+	use fastjet && tests+=" $(echo main{{211..214},216})" # 215 fails...
+	use root && tests+=" main143"
+	use hepmc3 && use lhapdf && tests+=" $(echo main{133,162})"
+	use fastjet && use hepmc3 && use lhapdf && tests+=" $(echo main161)"
+	# Other tests disabled due to missing dependencies
+
+	# use emake for parallel instead of long runmains
+	LD_LIBRARY_PATH="${S}/$(get_libdir):${LD_LIBRARY_PATH}" \
+	PYTHIA8DATA="../share/Pythia8/xmldoc/" \
+	LHAPDF_DATA_PATH="${WORKDIR}" \
+	emake ${tests}
+	emake clean
+}
+
+src_install() {
+	# make install is too broken, much easier to install manually
+	dobin bin/pythia8-config
+	doheader -r include/*
+	dolib.so lib/libpythia8.so
+	use static-libs && dolib.a lib/libpythia8.a
+	use lhapdf && dolib.so lib/libpythia8lhapdf6.so
+	insinto "${PYTHIADIR}"
+	doins -r share/Pythia8/tunes share/Pythia8/xmldoc share/Pythia8/pdfdata examples/Makefile.inc
+	dosym Pythia8 /usr/share/${PN}
+
+	newenvd - 99pythia8 <<- _EOF_
+		PYTHIA8DATA=${EPYTHIADIR}/xmldoc
+	_EOF_
+
+	dodoc AUTHORS GUIDELINES README
+
+	if use doc; then
+		dodoc -r share/Pythia8/pdfdoc/.
+		dodoc -r share/Pythia8/htmldoc/.
+	fi
+	if use examples; then
+		sed -i "s|include Makefile.inc|include ${EPYTHIADIR}|" \
+			examples/Makefile || die
+
+		doins -r examples
+		docompress -x /usr/share/doc/${PF}/examples
+	fi
+	if use python; then
+		local site_dir=$(python_get_sitedir)
+		insinto "${site_dir#${EPREFIX}}"
+		doins lib/pythia8.so
+	fi
+
+	# cleanup
+	unset PYTHIADIR EPYTHIADIR
+}
+
+pkg_postinstall() {
+	optfeature "python interface awkward array support" dev-python/awkward
+	optfeature "python interface vector support" dev-python/vector
+}
diff --git a/sci-physics/pythia/pythia-9999.ebuild b/sci-physics/pythia/pythia-9999.ebuild
index 2967483d6ed1e..3fc7af42a508b 100644
--- a/sci-physics/pythia/pythia-9999.ebuild
+++ b/sci-physics/pythia/pythia-9999.ebuild
@@ -6,7 +6,7 @@ EAPI=8
 PYTHON_COMPAT=( python3_{11..13} )
 inherit toolchain-funcs python-single-r1 optfeature
 
-MV=$(ver_cut 1-2)
+MY_PV=$(ver_cut 1-2)
 MY_P="${PN}${PV//./}"
 LHA_VER="6.2.1"
 
@@ -26,7 +26,7 @@ if [[ ${PV} == 9999 ]]; then
 	inherit git-r3
 	EGIT_REPO_URI="https://gitlab.com/Pythia8/releases"
 else
-	SRC_URI="https://pythia.org/download/${PN}${MV//./}/${MY_P}.tgz
+	SRC_URI="https://pythia.org/download/${PN}${MY_PV//./}/${MY_P}.tgz
 	${SRC_URI}"
 	KEYWORDS="~amd64 ~x86"
 	S="${WORKDIR}/${MY_P}"
@@ -34,29 +34,23 @@ fi
 
 LICENSE="GPL-2"
 SLOT="8"
-IUSE="doc examples fastjet +hepmc3 hepmc2 lhapdf root test zlib python highfive mpich rivet static-libs" # evtgen mg5mes rivet powheg
+IUSE="doc examples fastjet +hepmc3 lhapdf root test zlib python highfive mpich rivet static-libs" # evtgen mg5mes rivet powheg
 RESTRICT="!test? ( test )"
-REQUIRED_USE="
-	?? ( hepmc3 hepmc2 )
-	python? ( ${PYTHON_REQUIRED_USE} )
-"
+REQUIRED_USE="python? ( ${PYTHON_REQUIRED_USE} )"
 
 RDEPEND="
 	fastjet? ( sci-physics/fastjet )
 	hepmc3? ( sci-physics/hepmc:3= )
-	hepmc2? ( sci-physics/hepmc:2= )
 	lhapdf? ( sci-physics/lhapdf:= )
 	zlib? ( virtual/zlib:= )
 	highfive? (
 		sci-libs/highfive
 		sci-libs/hdf5[cxx]
 	)
-	rivet? (
-		>=sci-physics/rivet-4:*
-	)
+	rivet? ( >=sci-physics/rivet-4:* )
 	mpich? ( sys-cluster/mpich )
 	python? ( ${PYTHON_DEPS} )
-	"
+"
 DEPEND="${RDEPEND}"
 # ROOT is used only when building related tests
 BDEPEND="